it works :D

modules/system-settings/i18n-de.nix

@@ -0,0 +1,21 @@
+{
+	flake.modules.nixos.i18n-de = {
+		time.timeZone = "Europe/Berlin";
+
+		i18n.defaultLocale = "en_US.UTF-8";
+		i18n.extraLocales = [ "ja_JP.UTF-8/UTF-8" ];
+
+		i18n.extraLocaleSettings = {
+			LC_ADDRESS = "de_DE.UTF-8";
+			LC_IDENTIFICATION = "de_DE.UTF-8";
+			LC_MEASUREMENT = "de_DE.UTF-8";
+			LC_MONETARY = "de_DE.UTF-8";
+			LC_NAME = "de_DE.UTF-8";
+			LC_NUMERIC = "de_DE.UTF-8";
+			LC_PAPER = "de_DE.UTF-8";
+			LC_TELEPHONE = "de_DE.UTF-8";
+			LC_TIME = "de_DE.UTF-8";
+		};
+		console.keyMap = "de";
+	};
+}