diff --git a/src/common/bcp47.cpp b/src/common/bcp47.cpp
index b230164a0..d13165138 100644
--- a/src/common/bcp47.cpp
+++ b/src/common/bcp47.cpp
@@ -28,6 +28,12 @@
 
 namespace mtx::bcp47 {
 
+bool
+operator <(language_c::extension_t const &a,
+           language_c::extension_t const &b) {
+  return mtx::string::to_lower_ascii(a.singleton) < mtx::string::to_lower_ascii(b.singleton);
+}
+
 bool language_c::ms_disabled = false;
 
 language_c::extension_t::extension_t(std::string const &singleton_,
@@ -255,6 +261,8 @@ language_c::parse_extensions(std::string const &str) {
     else
       m_extensions.back().extensions.emplace_back(part);
 
+  std::sort(m_extensions.begin(), m_extensions.end());
+
   return validate_extensions();
 }
 
@@ -543,7 +551,8 @@ language_c::add_extension(extension_t const &extension) {
   for (auto const &extension_subtag : extension.extensions)
     extensions_lower.emplace_back(mtx::string::to_lower_ascii(extension_subtag));
 
-  m_extensions.emplace_back(extension_t{ mtx::string::to_lower_ascii(extension.singleton), extensions_lower });
+  auto cleaned_extension = extension_t{ mtx::string::to_lower_ascii(extension.singleton), extensions_lower };
+  m_extensions.insert(std::lower_bound(m_extensions.begin(), m_extensions.end(), cleaned_extension), cleaned_extension);
 
   m_formatted_up_to_date = false;
 
diff --git a/tests/unit/common/bcp47.cpp b/tests/unit/common/bcp47.cpp
index b41a4b904..6bdd28112 100644
--- a/tests/unit/common/bcp47.cpp
+++ b/tests/unit/common/bcp47.cpp
@@ -329,4 +329,9 @@ TEST(BCP47LanguageTags, ExtensionsRFC6497) {
   EXPECT_TRUE(mtx::bcp47::language_c::parse("ja-t-it-m0-xxx-v21a-2007").is_valid());
 }
 
+TEST(BCP47LanguageTags, ExtensionsFormatting) {
+  EXPECT_EQ("ja-t-test-u-attr-co-phonebk"s, mtx::bcp47::language_c::parse("ja-T-Test-U-AttR-CO-phoNEbk").format());
+  EXPECT_EQ("ja-t-test-u-attr-co-phonebk"s, mtx::bcp47::language_c::parse("ja-U-AttR-CO-phoNEbk-T-Test").format());
+}
+
 }