Unify translation files loading action

Since it is possible alternative WebUI could be coded in languages other than English,
WebUI must be able to load user-provided webui_en.qm.
At least one translated string must exist in order to generate an usable .qm file.

src/webui/webapplication.h

@@ -146,6 +146,7 @@ private:
     QMap<QString, TranslatedFile> m_translatedFiles;
     QString m_currentLocale;
     QTranslator m_translator;
+    bool m_translationFileLoaded = false;
 
     bool m_isLocalAuthEnabled;
     bool m_isAuthSubnetWhitelistEnabled;