diff --git a/Cargo.lock b/Cargo.lock
index 8253fa4..9d9a8fa 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -212,6 +212,7 @@ dependencies = [
  "libc",
  "num-integer",
  "num-traits",
+ "pure-rust-locales",
  "time",
  "winapi 0.3.9",
 ]
@@ -399,6 +400,16 @@ dependencies = [
  "lazy_static",
 ]
 
+[[package]]
+name = "cstr_core"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "644828c273c063ab0d39486ba42a5d1f3a499d35529c759e763a9c6cb8a0fb08"
+dependencies = [
+ "cty",
+ "memchr",
+]
+
 [[package]]
 name = "ctor"
 version = "0.1.20"
@@ -409,6 +420,12 @@ dependencies = [
  "syn 1.0.67",
 ]
 
+[[package]]
+name = "cty"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b365fabc795046672053e29c954733ec3b05e4be654ab130fe8f1f94d7051f35"
+
 [[package]]
 name = "dbus"
 version = "0.9.1"
@@ -851,6 +868,7 @@ dependencies = [
  "log",
  "rand 0.8.3",
  "regex",
+ "sys-locale",
  "thiserror",
 ]
 
@@ -2040,6 +2058,12 @@ dependencies = [
  "unicode-xid 0.2.1",
 ]
 
+[[package]]
+name = "pure-rust-locales"
+version = "0.5.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b45c49fc4f91f35bae654f85ebb3a44d60ac64f11b3166ffa609def390c732d8"
+
 [[package]]
 name = "quote"
 version = "0.3.15"
@@ -2577,6 +2601,19 @@ dependencies = [
  "unicode-xid 0.0.4",
 ]
 
+[[package]]
+name = "sys-locale"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91f89ebb59fa30d4f65fafc2d68e94f6975256fd87e812dd99cb6e020c8563df"
+dependencies = [
+ "cc",
+ "cstr_core",
+ "libc",
+ "web-sys",
+ "winapi 0.3.9",
+]
+
 [[package]]
 name = "tempdir"
 version = "0.3.7"
diff --git a/espanso-render/Cargo.toml b/espanso-render/Cargo.toml
index 3940d0d..2244e8a 100644
--- a/espanso-render/Cargo.toml
+++ b/espanso-render/Cargo.toml
@@ -10,6 +10,7 @@ anyhow = "1.0.38"
 thiserror = "1.0.23"
 regex = "1.4.3"
 lazy_static = "1.4.0"
-chrono = "0.4.19"
+chrono = {version = "0.4.19", features=["unstable-locales"]}
 enum-as-inner = "0.3.3"
-rand = "0.8.3"
\ No newline at end of file
+rand = "0.8.3"
+sys-locale = "0.1.0"
\ No newline at end of file
diff --git a/espanso-render/src/extension/date.rs b/espanso-render/src/extension/date.rs
index 5cb0812..bd1daf7 100644
--- a/espanso-render/src/extension/date.rs
+++ b/espanso-render/src/extension/date.rs
@@ -17,22 +17,30 @@
  * along with espanso.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-use chrono::{DateTime, Duration, Local};
+use chrono::{DateTime, Duration, Local, Locale};
 
 use crate::{Extension, ExtensionOutput, ExtensionResult, Number, Params, Value};
 
-pub struct DateExtension {
+pub trait LocaleProvider {
+  fn get_system_locale(&self) -> String;
+}
+
+pub struct DateExtension<'a> {
   fixed_date: Option<DateTime<Local>>,
+  locale_provider: &'a dyn LocaleProvider,
 }
 
 #[allow(clippy::new_without_default)]
-impl DateExtension {
-  pub fn new() -> Self {
-    Self { fixed_date: None }
+impl<'a> DateExtension<'a> {
+  pub fn new(locale_provider: &'a dyn LocaleProvider) -> Self {
+    Self {
+      fixed_date: None,
+      locale_provider,
+    }
   }
 }
 
-impl Extension for DateExtension {
+impl<'a> Extension for DateExtension<'a> {
   fn name(&self) -> &str {
     "date"
   }
@@ -53,9 +61,14 @@ impl Extension for DateExtension {
     }
 
     let format = params.get("format");
+    let locale = params
+      .get("locale")
+      .and_then(|val| val.as_string())
+      .map(String::from)
+      .unwrap_or_else(|| self.locale_provider.get_system_locale());
 
     let date = if let Some(Value::String(format)) = format {
-      now.format(format).to_string()
+      DateExtension::format_date_with_locale_string(now, format, &locale)
     } else {
       now.to_rfc2822()
     };
@@ -64,7 +77,7 @@ impl Extension for DateExtension {
   }
 }
 
-impl DateExtension {
+impl<'a> DateExtension<'a> {
   fn get_date(&self) -> DateTime<Local> {
     if let Some(fixed_date) = self.fixed_date {
       fixed_date
@@ -72,6 +85,339 @@ impl DateExtension {
       Local::now()
     }
   }
+
+  fn format_date_with_locale(date: DateTime<Local>, format: &str, locale: Locale) -> String {
+    date.format_localized(format, locale).to_string()
+  }
+
+  fn format_date_with_locale_string(
+    date: DateTime<Local>,
+    format: &str,
+    locale_str: &str,
+  ) -> String {
+    let locale = convert_locale_string_to_locale(locale_str).unwrap_or(Locale::en_US);
+    Self::format_date_with_locale(date, format, locale)
+  }
+}
+
+fn convert_locale_string_to_locale(locale_str: &str) -> Option<Locale> {
+  match locale_str {
+    "aa-DJ" => Some(Locale::aa_DJ),
+    "aa-ER" => Some(Locale::aa_ER),
+    "aa-ET" => Some(Locale::aa_ET),
+    "af-ZA" => Some(Locale::af_ZA),
+    "agr-PE" => Some(Locale::agr_PE),
+    "ak-GH" => Some(Locale::ak_GH),
+    "am-ET" => Some(Locale::am_ET),
+    "an-ES" => Some(Locale::an_ES),
+    "anp-IN" => Some(Locale::anp_IN),
+    "ar-AE" => Some(Locale::ar_AE),
+    "ar-BH" => Some(Locale::ar_BH),
+    "ar-DZ" => Some(Locale::ar_DZ),
+    "ar-EG" => Some(Locale::ar_EG),
+    "ar-IN" => Some(Locale::ar_IN),
+    "ar-IQ" => Some(Locale::ar_IQ),
+    "ar-JO" => Some(Locale::ar_JO),
+    "ar-KW" => Some(Locale::ar_KW),
+    "ar-LB" => Some(Locale::ar_LB),
+    "ar-LY" => Some(Locale::ar_LY),
+    "ar-MA" => Some(Locale::ar_MA),
+    "ar-OM" => Some(Locale::ar_OM),
+    "ar-QA" => Some(Locale::ar_QA),
+    "ar-SA" => Some(Locale::ar_SA),
+    "ar-SD" => Some(Locale::ar_SD),
+    "ar-SS" => Some(Locale::ar_SS),
+    "ar-SY" => Some(Locale::ar_SY),
+    "ar-TN" => Some(Locale::ar_TN),
+    "ar-YE" => Some(Locale::ar_YE),
+    "as-IN" => Some(Locale::as_IN),
+    "ast-ES" => Some(Locale::ast_ES),
+    "ayc-PE" => Some(Locale::ayc_PE),
+    "az-AZ" => Some(Locale::az_AZ),
+    "az-IR" => Some(Locale::az_IR),
+    "be-BY" => Some(Locale::be_BY),
+    "bem-ZM" => Some(Locale::bem_ZM),
+    "ber-DZ" => Some(Locale::ber_DZ),
+    "ber-MA" => Some(Locale::ber_MA),
+    "bg-BG" => Some(Locale::bg_BG),
+    "bhb-IN" => Some(Locale::bhb_IN),
+    "bho-IN" => Some(Locale::bho_IN),
+    "bho-NP" => Some(Locale::bho_NP),
+    "bi-VU" => Some(Locale::bi_VU),
+    "bn-BD" => Some(Locale::bn_BD),
+    "bn-IN" => Some(Locale::bn_IN),
+    "bo-CN" => Some(Locale::bo_CN),
+    "bo-IN" => Some(Locale::bo_IN),
+    "br-FR" => Some(Locale::br_FR),
+    "brx-IN" => Some(Locale::brx_IN),
+    "bs-BA" => Some(Locale::bs_BA),
+    "byn-ER" => Some(Locale::byn_ER),
+    "ca-AD" => Some(Locale::ca_AD),
+    "ca-ES" => Some(Locale::ca_ES),
+    "ca-FR" => Some(Locale::ca_FR),
+    "ca-IT" => Some(Locale::ca_IT),
+    "ce-RU" => Some(Locale::ce_RU),
+    "chr-US" => Some(Locale::chr_US),
+    "cmn-TW" => Some(Locale::cmn_TW),
+    "crh-UA" => Some(Locale::crh_UA),
+    "cs-CZ" => Some(Locale::cs_CZ),
+    "csb-PL" => Some(Locale::csb_PL),
+    "cv-RU" => Some(Locale::cv_RU),
+    "cy-GB" => Some(Locale::cy_GB),
+    "da-DK" => Some(Locale::da_DK),
+    "de-AT" => Some(Locale::de_AT),
+    "de-BE" => Some(Locale::de_BE),
+    "de-CH" => Some(Locale::de_CH),
+    "de-DE" => Some(Locale::de_DE),
+    "de-IT" => Some(Locale::de_IT),
+    "de-LI" => Some(Locale::de_LI),
+    "de-LU" => Some(Locale::de_LU),
+    "doi-IN" => Some(Locale::doi_IN),
+    "dsb-DE" => Some(Locale::dsb_DE),
+    "dv-MV" => Some(Locale::dv_MV),
+    "dz-BT" => Some(Locale::dz_BT),
+    "el-CY" => Some(Locale::el_CY),
+    "el-GR" => Some(Locale::el_GR),
+    "en-AG" => Some(Locale::en_AG),
+    "en-AU" => Some(Locale::en_AU),
+    "en-BW" => Some(Locale::en_BW),
+    "en-CA" => Some(Locale::en_CA),
+    "en-DK" => Some(Locale::en_DK),
+    "en-GB" => Some(Locale::en_GB),
+    "en-HK" => Some(Locale::en_HK),
+    "en-IE" => Some(Locale::en_IE),
+    "en-IL" => Some(Locale::en_IL),
+    "en-IN" => Some(Locale::en_IN),
+    "en-NG" => Some(Locale::en_NG),
+    "en-NZ" => Some(Locale::en_NZ),
+    "en-PH" => Some(Locale::en_PH),
+    "en-SC" => Some(Locale::en_SC),
+    "en-SG" => Some(Locale::en_SG),
+    "en-US" => Some(Locale::en_US),
+    "en-ZA" => Some(Locale::en_ZA),
+    "en-ZM" => Some(Locale::en_ZM),
+    "en-ZW" => Some(Locale::en_ZW),
+    "eo" => Some(Locale::eo),
+    "es-AR" => Some(Locale::es_AR),
+    "es-BO" => Some(Locale::es_BO),
+    "es-CL" => Some(Locale::es_CL),
+    "es-CO" => Some(Locale::es_CO),
+    "es-CR" => Some(Locale::es_CR),
+    "es-CU" => Some(Locale::es_CU),
+    "es-DO" => Some(Locale::es_DO),
+    "es-EC" => Some(Locale::es_EC),
+    "es-ES" => Some(Locale::es_ES),
+    "es-GT" => Some(Locale::es_GT),
+    "es-HN" => Some(Locale::es_HN),
+    "es-MX" => Some(Locale::es_MX),
+    "es-NI" => Some(Locale::es_NI),
+    "es-PA" => Some(Locale::es_PA),
+    "es-PE" => Some(Locale::es_PE),
+    "es-PR" => Some(Locale::es_PR),
+    "es-PY" => Some(Locale::es_PY),
+    "es-SV" => Some(Locale::es_SV),
+    "es-US" => Some(Locale::es_US),
+    "es-UY" => Some(Locale::es_UY),
+    "es-VE" => Some(Locale::es_VE),
+    "et-EE" => Some(Locale::et_EE),
+    "eu-ES" => Some(Locale::eu_ES),
+    "fa-IR" => Some(Locale::fa_IR),
+    "ff-SN" => Some(Locale::ff_SN),
+    "fi-FI" => Some(Locale::fi_FI),
+    "fil-PH" => Some(Locale::fil_PH),
+    "fo-FO" => Some(Locale::fo_FO),
+    "fr-BE" => Some(Locale::fr_BE),
+    "fr-CA" => Some(Locale::fr_CA),
+    "fr-CH" => Some(Locale::fr_CH),
+    "fr-FR" => Some(Locale::fr_FR),
+    "fr-LU" => Some(Locale::fr_LU),
+    "fur-IT" => Some(Locale::fur_IT),
+    "fy-DE" => Some(Locale::fy_DE),
+    "fy-NL" => Some(Locale::fy_NL),
+    "ga-IE" => Some(Locale::ga_IE),
+    "gd-GB" => Some(Locale::gd_GB),
+    "gez-ER" => Some(Locale::gez_ER),
+    "gez-ET" => Some(Locale::gez_ET),
+    "gl-ES" => Some(Locale::gl_ES),
+    "gu-IN" => Some(Locale::gu_IN),
+    "gv-GB" => Some(Locale::gv_GB),
+    "ha-NG" => Some(Locale::ha_NG),
+    "hak-TW" => Some(Locale::hak_TW),
+    "he-IL" => Some(Locale::he_IL),
+    "hi-IN" => Some(Locale::hi_IN),
+    "hif-FJ" => Some(Locale::hif_FJ),
+    "hne-IN" => Some(Locale::hne_IN),
+    "hr-HR" => Some(Locale::hr_HR),
+    "hsb-DE" => Some(Locale::hsb_DE),
+    "ht-HT" => Some(Locale::ht_HT),
+    "hu-HU" => Some(Locale::hu_HU),
+    "hy-AM" => Some(Locale::hy_AM),
+    "ia-FR" => Some(Locale::ia_FR),
+    "id-ID" => Some(Locale::id_ID),
+    "ig-NG" => Some(Locale::ig_NG),
+    "ik-CA" => Some(Locale::ik_CA),
+    "is-IS" => Some(Locale::is_IS),
+    "it-CH" => Some(Locale::it_CH),
+    "it-IT" => Some(Locale::it_IT),
+    "iu-CA" => Some(Locale::iu_CA),
+    "ja-JP" => Some(Locale::ja_JP),
+    "ka-GE" => Some(Locale::ka_GE),
+    "kab-DZ" => Some(Locale::kab_DZ),
+    "kk-KZ" => Some(Locale::kk_KZ),
+    "kl-GL" => Some(Locale::kl_GL),
+    "km-KH" => Some(Locale::km_KH),
+    "kn-IN" => Some(Locale::kn_IN),
+    "ko-KR" => Some(Locale::ko_KR),
+    "kok-IN" => Some(Locale::kok_IN),
+    "ks-IN" => Some(Locale::ks_IN),
+    "ku-TR" => Some(Locale::ku_TR),
+    "kw-GB" => Some(Locale::kw_GB),
+    "ky-KG" => Some(Locale::ky_KG),
+    "lb-LU" => Some(Locale::lb_LU),
+    "lg-UG" => Some(Locale::lg_UG),
+    "li-BE" => Some(Locale::li_BE),
+    "li-NL" => Some(Locale::li_NL),
+    "lij-IT" => Some(Locale::lij_IT),
+    "ln-CD" => Some(Locale::ln_CD),
+    "lo-LA" => Some(Locale::lo_LA),
+    "lt-LT" => Some(Locale::lt_LT),
+    "lv-LV" => Some(Locale::lv_LV),
+    "lzh-TW" => Some(Locale::lzh_TW),
+    "mag-IN" => Some(Locale::mag_IN),
+    "mai-IN" => Some(Locale::mai_IN),
+    "mai-NP" => Some(Locale::mai_NP),
+    "mfe-MU" => Some(Locale::mfe_MU),
+    "mg-MG" => Some(Locale::mg_MG),
+    "mhr-RU" => Some(Locale::mhr_RU),
+    "mi-NZ" => Some(Locale::mi_NZ),
+    "miq-NI" => Some(Locale::miq_NI),
+    "mjw-IN" => Some(Locale::mjw_IN),
+    "mk-MK" => Some(Locale::mk_MK),
+    "ml-IN" => Some(Locale::ml_IN),
+    "mn-MN" => Some(Locale::mn_MN),
+    "mni-IN" => Some(Locale::mni_IN),
+    "mnw-MM" => Some(Locale::mnw_MM),
+    "mr-IN" => Some(Locale::mr_IN),
+    "ms-MY" => Some(Locale::ms_MY),
+    "mt-MT" => Some(Locale::mt_MT),
+    "my-MM" => Some(Locale::my_MM),
+    "nan-TW" => Some(Locale::nan_TW),
+    "nb-NO" => Some(Locale::nb_NO),
+    "nds-DE" => Some(Locale::nds_DE),
+    "nds-NL" => Some(Locale::nds_NL),
+    "ne-NP" => Some(Locale::ne_NP),
+    "nhn-MX" => Some(Locale::nhn_MX),
+    "niu-NU" => Some(Locale::niu_NU),
+    "niu-NZ" => Some(Locale::niu_NZ),
+    "nl-AW" => Some(Locale::nl_AW),
+    "nl-BE" => Some(Locale::nl_BE),
+    "nl-NL" => Some(Locale::nl_NL),
+    "nn-NO" => Some(Locale::nn_NO),
+    "nr-ZA" => Some(Locale::nr_ZA),
+    "nso-ZA" => Some(Locale::nso_ZA),
+    "oc-FR" => Some(Locale::oc_FR),
+    "om-ET" => Some(Locale::om_ET),
+    "om-KE" => Some(Locale::om_KE),
+    "or-IN" => Some(Locale::or_IN),
+    "os-RU" => Some(Locale::os_RU),
+    "pa-IN" => Some(Locale::pa_IN),
+    "pa-PK" => Some(Locale::pa_PK),
+    "pap-AW" => Some(Locale::pap_AW),
+    "pap-CW" => Some(Locale::pap_CW),
+    "pl-PL" => Some(Locale::pl_PL),
+    "ps-AF" => Some(Locale::ps_AF),
+    "pt-BR" => Some(Locale::pt_BR),
+    "pt-PT" => Some(Locale::pt_PT),
+    "quz-PE" => Some(Locale::quz_PE),
+    "raj-IN" => Some(Locale::raj_IN),
+    "ro-RO" => Some(Locale::ro_RO),
+    "ru-RU" => Some(Locale::ru_RU),
+    "ru-UA" => Some(Locale::ru_UA),
+    "rw-RW" => Some(Locale::rw_RW),
+    "sa-IN" => Some(Locale::sa_IN),
+    "sah-RU" => Some(Locale::sah_RU),
+    "sat-IN" => Some(Locale::sat_IN),
+    "sc-IT" => Some(Locale::sc_IT),
+    "sd-IN" => Some(Locale::sd_IN),
+    "se-NO" => Some(Locale::se_NO),
+    "sgs-LT" => Some(Locale::sgs_LT),
+    "shn-MM" => Some(Locale::shn_MM),
+    "shs-CA" => Some(Locale::shs_CA),
+    "si-LK" => Some(Locale::si_LK),
+    "sid-ET" => Some(Locale::sid_ET),
+    "sk-SK" => Some(Locale::sk_SK),
+    "sl-SI" => Some(Locale::sl_SI),
+    "sm-WS" => Some(Locale::sm_WS),
+    "so-DJ" => Some(Locale::so_DJ),
+    "so-ET" => Some(Locale::so_ET),
+    "so-KE" => Some(Locale::so_KE),
+    "so-SO" => Some(Locale::so_SO),
+    "sq-AL" => Some(Locale::sq_AL),
+    "sq-MK" => Some(Locale::sq_MK),
+    "sr-ME" => Some(Locale::sr_ME),
+    "sr-RS" => Some(Locale::sr_RS),
+    "ss-ZA" => Some(Locale::ss_ZA),
+    "st-ZA" => Some(Locale::st_ZA),
+    "sv-FI" => Some(Locale::sv_FI),
+    "sv-SE" => Some(Locale::sv_SE),
+    "sw-KE" => Some(Locale::sw_KE),
+    "sw-TZ" => Some(Locale::sw_TZ),
+    "szl-PL" => Some(Locale::szl_PL),
+    "ta-IN" => Some(Locale::ta_IN),
+    "ta-LK" => Some(Locale::ta_LK),
+    "tcy-IN" => Some(Locale::tcy_IN),
+    "te-IN" => Some(Locale::te_IN),
+    "tg-TJ" => Some(Locale::tg_TJ),
+    "th-TH" => Some(Locale::th_TH),
+    "the-NP" => Some(Locale::the_NP),
+    "ti-ER" => Some(Locale::ti_ER),
+    "ti-ET" => Some(Locale::ti_ET),
+    "tig-ER" => Some(Locale::tig_ER),
+    "tk-TM" => Some(Locale::tk_TM),
+    "tl-PH" => Some(Locale::tl_PH),
+    "tn-ZA" => Some(Locale::tn_ZA),
+    "to-TO" => Some(Locale::to_TO),
+    "tpi-PG" => Some(Locale::tpi_PG),
+    "tr-CY" => Some(Locale::tr_CY),
+    "tr-TR" => Some(Locale::tr_TR),
+    "ts-ZA" => Some(Locale::ts_ZA),
+    "tt-RU" => Some(Locale::tt_RU),
+    "ug-CN" => Some(Locale::ug_CN),
+    "uk-UA" => Some(Locale::uk_UA),
+    "unm-US" => Some(Locale::unm_US),
+    "ur-IN" => Some(Locale::ur_IN),
+    "ur-PK" => Some(Locale::ur_PK),
+    "uz-UZ" => Some(Locale::uz_UZ),
+    "ve-ZA" => Some(Locale::ve_ZA),
+    "vi-VN" => Some(Locale::vi_VN),
+    "wa-BE" => Some(Locale::wa_BE),
+    "wae-CH" => Some(Locale::wae_CH),
+    "wal-ET" => Some(Locale::wal_ET),
+    "wo-SN" => Some(Locale::wo_SN),
+    "xh-ZA" => Some(Locale::xh_ZA),
+    "yi-US" => Some(Locale::yi_US),
+    "yo-NG" => Some(Locale::yo_NG),
+    "yue-HK" => Some(Locale::yue_HK),
+    "yuw-PG" => Some(Locale::yuw_PG),
+    "zh-CN" => Some(Locale::zh_CN),
+    "zh-HK" => Some(Locale::zh_HK),
+    "zh-SG" => Some(Locale::zh_SG),
+    "zh-TW" => Some(Locale::zh_TW),
+    "zu-ZA" => Some(Locale::zu_ZA),
+    _ => None,
+  }
+}
+
+pub struct DefaultLocaleProvider {}
+impl LocaleProvider for DefaultLocaleProvider {
+  fn get_system_locale(&self) -> String {
+    sys_locale::get_locale().unwrap_or_else(|| String::from("en-US"))
+  }
+}
+impl DefaultLocaleProvider {
+  pub fn new() -> Self {
+    Self {}
+  }
 }
 
 #[cfg(test)]
@@ -79,9 +425,30 @@ mod tests {
   use super::*;
   use chrono::offset::TimeZone;
 
+  struct MockLocaleProvider {
+    locale: String,
+  }
+  impl LocaleProvider for MockLocaleProvider {
+    fn get_system_locale(&self) -> String {
+      self.locale.clone()
+    }
+  }
+  impl MockLocaleProvider {
+    pub fn new() -> Self {
+      Self {
+        locale: "en-US".to_string(),
+      }
+    }
+
+    pub fn new_with_locale(locale: String) -> Self {
+      Self { locale }
+    }
+  }
+
   #[test]
   fn date_formatted_correctly() {
-    let mut extension = DateExtension::new();
+    let locale_provider = MockLocaleProvider::new();
+    let mut extension = DateExtension::new(&locale_provider);
     extension.fixed_date = Some(Local.ymd(2014, 7, 8).and_hms(9, 10, 11));
 
     let param = vec![("format".to_string(), Value::String("%H:%M:%S".to_string()))]
@@ -98,7 +465,8 @@ mod tests {
 
   #[test]
   fn offset_works_correctly() {
-    let mut extension = DateExtension::new();
+    let locale_provider = MockLocaleProvider::new();
+    let mut extension = DateExtension::new(&locale_provider);
     extension.fixed_date = Some(Local.ymd(2014, 7, 8).and_hms(9, 10, 11));
 
     let param = vec![
@@ -115,4 +483,61 @@ mod tests {
       ExtensionOutput::Single("10:10:11".to_string())
     );
   }
+
+  #[test]
+  fn default_locale_works_correctly() {
+    let locale_provider = MockLocaleProvider::new_with_locale("it-IT".to_string());
+    let mut extension = DateExtension::new(&locale_provider);
+    extension.fixed_date = Some(Local.ymd(2014, 7, 8).and_hms(9, 10, 11));
+
+    let param = vec![("format".to_string(), Value::String("%A".to_string()))]
+      .into_iter()
+      .collect::<Params>();
+    assert_eq!(
+      extension
+        .calculate(&Default::default(), &Default::default(), &param)
+        .into_success()
+        .unwrap(),
+      ExtensionOutput::Single("martedì".to_string())
+    );
+  }
+
+  #[test]
+  fn invalid_locale_should_default_to_en_us() {
+    let locale_provider = MockLocaleProvider::new_with_locale("invalid".to_string());
+    let mut extension = DateExtension::new(&locale_provider);
+    extension.fixed_date = Some(Local.ymd(2014, 7, 8).and_hms(9, 10, 11));
+
+    let param = vec![("format".to_string(), Value::String("%A".to_string()))]
+      .into_iter()
+      .collect::<Params>();
+    assert_eq!(
+      extension
+        .calculate(&Default::default(), &Default::default(), &param)
+        .into_success()
+        .unwrap(),
+      ExtensionOutput::Single("Tuesday".to_string())
+    );
+  }
+
+  #[test]
+  fn override_locale() {
+    let locale_provider = MockLocaleProvider::new();
+    let mut extension = DateExtension::new(&locale_provider);
+    extension.fixed_date = Some(Local.ymd(2014, 7, 8).and_hms(9, 10, 11));
+
+    let param = vec![
+      ("format".to_string(), Value::String("%A".to_string())),
+      ("locale".to_string(), Value::String("it-IT".to_string())),
+    ]
+    .into_iter()
+    .collect::<Params>();
+    assert_eq!(
+      extension
+        .calculate(&Default::default(), &Default::default(), &param)
+        .into_success()
+        .unwrap(),
+      ExtensionOutput::Single("martedì".to_string())
+    );
+  }
 }