Exactly what the title says: instead of having to delete the config/messages.yml when changing language, run the same thing that migrates config options upon version upgrade.

What is supposed to happen if the automatically (incorrectly) detected langauge is different than the language of the currently used messages.yml? Will new messages (from updates) be in the right language?

the values will stay the same, apart from new values & comments. You can set ForcedLanguage to the right language

While migration would effectively update all the comments in both the files, the values (especially in the messages.yml) would stay the same. The only way to know if the language has been updated is to store it, with a soft-reload (/discord reload) it would be easy just to store it in memory, but if the user were to restart the server completely, we would need to store the language (on disable) to a file/external storage medium (Just relying on the user soft-reloading is not very user friendly).

Having a command to change the language would likely be much simpler.

This is very unlikely to be added to DiscordSRV1, thus adding v2.0.0 milestone

/discord setlang <language> was added a while back, which accomplishes the same thing, except it generates completely new files & saves a backup of the old ones