Unlocalised strings for non-English languages show up as localisation keys when the browser sends a non-English Accept-Language header, affecting both auto-detected and set-via-preference-pane webapp languages

[akemin-dayo]

Steps to reproduce

• Use a browser that sends a non-English Accept-Language header. For instance, ja-JP or ja in my case. ※ You can also set this using Chromium DevTools.
• Open a version of the webapp that contains commit e59d035. Both master (7392447) and stable 5.5 are affected, including the demo instance as of this writing. (Also affects my backport which includes the same commit, but that's not really relevant here.)
• Open the side menu and notice that strings that unlocalised strings show up not in English, but in their key form.
• Open the preference pane and change the language to English.
• Now change it back to whatever language you had before. (※ Japanese in my case)
• Previously untranslated keys will now show up correctly as English strings.
• Refresh the webapp.
• Observe that untranslated strings are in their key form again.

Expected result

• Untranslated strings should appear as English strings, not as localisation keys.

Actual result

• Localisation keys appear instead of English strings for untranslated strings.

Context

BlueMap Version: 7392447
※ Also affects the demo instance: https://bluecolored.de/bluemap/

Screenshots (JP)

Screenshots (KR)

[akemin-dayo]

Can confirm that #632 (ce7e477) fixes the issue.