Improve localization logic

Make it so that the language behavior is more stable. Try the localizations in the order of configuration. This will allow e.g. a reasonable default language to be used for a missing translation. Adding a new language through config should have a stable default.

- [ ] arrange the localizations so that they are always in the order of configured languages
- [ ] change logic to take either the asked language if it exists, or the first available language in the order

See also #1334

Start by tracing where configured languages are assembled and where a requested translation falls back; the issue names no files or tests. Done means localization order is deterministic, follows configuration, and selects the requested language when available or the first configured language otherwise, including after adding a language through configuration.

Written by the indexing model from the issue text.