I'm on OMV 4.1.23-1 and access it from Firefox 68.0.2 (64-Bit, Windows and Linux).
The Login Window offers a GUI language selector. I configured German as preferred language in Firefox (in one test German only, in other tests German as primary language, English (en) and English (en-us) as fallback languages).
OMV defaults to German after logging out from OMV and then logging back in. However, after each start of Firefox, OMV's language selector defaults to English again.
This happens in Firefox only. Opera and Chrome work fine.
I already tried to investigate the problem in the source code and at runtime:
Login.js with the language field calls LanguageCombo.js calls i18n.js. I18n.js stores the detected language in the cookie X-OPENMEDIAVAULT-LOCALE, which is empty at startup (for privacy reasons I configured Firefox to flush all cookies at the end). Therefore i18n.getlocale() returns an empty value, causing LanguageComboBox.js to default to English.
I think to overcome this issue OMV.util.i18n.getLocale() should be called at initialization before displaying the Login window.
Despite development experience I have not developed in JS yet, so I might be wrong.
What do you think, shall I file a bug in the tracker or how can we overcome this issue?