I am pretty sure that the language selector modal ("Editing language") comes from FoF Discussion Language, and it seems that it's missing some settings, so please check that (for example, you must select both country and language in order for the selector to work properly).
If you are looking for translation support, then you must enable the translation extensions available, not FoF Discussion Language extension which is purely just for setting custom language labels to discussion (for example, if you want to run a multilingual forum). The language extensions are under the "Languages" category in your admin panel.
I don't know what settings are you using, so I can't help you, but if you need help, it might be better to ask this in the appropriate thread. You can also attach screenshot of your FoF Discussion Language extension settings. If you don't need language labels for discussions, you can just disable the extension and your forum should work normally again.
If you believe that you have encountered some error that is FreeFlarum (/flarum.cloud) related (eg.: a 5xx error occurs), please tell me your forum name in our thread and I will check your forum logs for potential errors (eg.: the extension's settings failed to save, an error was reported, etc...).