Support for preserving user-defined languages

### Describe the problem

Since https://github.com/WeblateOrg/weblate/pull/13534 any languages which are aliased in our language data are merged based on the alias upon upgrade. This makes it impossible for users to maintain such a language they want for whatever reason.

### Describe the solution you would like

The manually created languages would be kept. The automatically created ones (either created by previous Weblate versions during migration or via automatic creating while parsing) should be still migrated as they are now.

### Describe alternatives you have considered

Disabling [UPDATE_LANGUAGES](https://docs.weblate.org/en/latest/admin/config.html#std-setting-UPDATE_LANGUAGES) is a workaround, but that prohibits all language updates.

### Screenshots

_No response_

### Additional context

_No response_