良い感じに

locales/index.js

@@ -11,13 +11,13 @@ const loadLang = lang => yaml.safeLoad(
 const native = loadLang('ja-JP');
 
 const langs = {
-	'de': loadLang('de'),
-	'en': loadLang('en'),
-	'fr': loadLang('fr'),
-	'ja': native,
+	'de-DE': loadLang('de-DE'),
+	'en-US': loadLang('en-US'),
+	'fr-FR': loadLang('fr-FR'),
+	'ja-JP': native,
 	'ja-KS': loadLang('ja-KS'),
-	'pl': loadLang('pl'),
-	'es': loadLang('es')
+	'pl-PL': loadLang('pl-PL'),
+	'es-ES': loadLang('es-ES')
 };
 
 Object.values(langs).forEach(locale => {

src/client/app/boot.js

@@ -38,12 +38,18 @@
 	//#endregion
 
 	//#region Detect the user language
-	let lang = navigator.language;
+	let lang = null;
 
-	if (!LANGS.includes(lang)) lang = lang.split('-')[0];
+	if (LANGS.includes(navigator.language)) {
+		lang = navigator.language;
+	} else {
+		lang = LANGS.find(x => x.split('-')[0] == lang);
 
-	// The default language is English
-	if (!LANGS.includes(lang)) lang = 'en';
+		if (lang == null) {
+			// Fallback
+			lang = 'en-US';
+		}
+	}
 
 	if (settings) {
 		if (settings.device.lang) lang = settings.device.lang;