diff --git a/internal/api/oidc_redirect.go b/internal/api/oidc_redirect.go
index c39d2a0cb..92f307175 100644
--- a/internal/api/oidc_redirect.go
+++ b/internal/api/oidc_redirect.go
@@ -132,9 +132,7 @@ func OIDCRedirect(router *gin.RouterGroup) {
 			user.Details().UserGender = clean.Name(string(userInfo.GetGender()))
 
 			// Set UI locale.
-			if locale, _, _ := userInfo.GetLocale().Raw(); len(locale.String()) == 2 {
-				user.Settings().UILanguage = locale.String()
-			}
+			user.Settings().UILanguage = clean.Locale(userInfo.GetLocale().String(), "")
 
 			// Set UI timezone.
 			user.Settings().UITimeZone = userInfo.GetZoneinfo()
diff --git a/pkg/clean/locale.go b/pkg/clean/locale.go
new file mode 100644
index 000000000..ef14596b9
--- /dev/null
+++ b/pkg/clean/locale.go
@@ -0,0 +1,21 @@
+package clean
+
+import "strings"
+
+// Locale returns the normalized locale string in POSIX format with underscore, or the default locale otherwise.
+// See https://en.wikipedia.org/wiki/Locale_(computer_software) for details.
+func Locale(locale, defaultLocale string) string {
+	if locale == "" {
+		return defaultLocale
+	}
+
+	locale, _, _ = strings.Cut(strings.Replace(locale, "-", "_", 1), ".")
+
+	if l := len(locale); l == 2 {
+		return strings.ToLower(locale)
+	} else if l == 5 && locale[2] == '_' {
+		return strings.ToLower(locale[:2]) + "_" + strings.ToUpper(locale[3:])
+	}
+
+	return defaultLocale
+}
diff --git a/pkg/clean/locale_test.go b/pkg/clean/locale_test.go
new file mode 100644
index 000000000..0c5fbec39
--- /dev/null
+++ b/pkg/clean/locale_test.go
@@ -0,0 +1,32 @@
+package clean
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestLocale(t *testing.T) {
+	t.Run("Empty", func(t *testing.T) {
+		assert.Equal(t, "", Locale("", ""))
+		assert.Equal(t, "de", Locale("", "de"))
+		assert.Equal(t, "und", Locale("", "und"))
+	})
+	t.Run("Language", func(t *testing.T) {
+		assert.Equal(t, "de", Locale("de", ""))
+		assert.Equal(t, "", Locale("und", ""))
+		assert.Equal(t, "de", Locale("und", "de"))
+		assert.Equal(t, "cs", Locale("cs", "und"))
+	})
+	t.Run("Territory", func(t *testing.T) {
+		assert.Equal(t, "cs_CZ", Locale("cs_CZ", ""))
+		assert.Equal(t, "cs_CZ", Locale("cs-CZ", ""))
+		assert.Equal(t, "cs_CZ", Locale("cs_cz", ""))
+		assert.Equal(t, "cs_CZ", Locale("cs-cz", ""))
+		assert.Equal(t, "cs_CZ", Locale("Cs_cz", ""))
+		assert.Equal(t, "cs_CZ", Locale("Cs-cz", ""))
+		assert.Equal(t, "cs_CZ", Locale("cs_CZ", "und"))
+		assert.Equal(t, "cs_CZ", Locale("cs-CZ", "und"))
+		assert.Equal(t, "und", Locale("cs-CZX", "und"))
+	})
+}