If you regularly post in multiple languages, you have to select them all in your user settings.
However, having multiple languages selected will always display the same language list in the same order, even if you don't post in that language.
For example, I post mostly in french but sometimes in english as well. Well, the lemmy front-end always puts english at the start of the list so every time I post in french, I need to re-select it which is kind of annoying. This add-on does it for you!
The other part is, lemmy hides content from you unless you have the language enabled in your settings, and so, I have all languages enabled just so I can see all the content. And that makes navigating the language list really unwieldy without a default option
Yes, it's exactly that, I explained it in another reply
I also agree it should behave like that by default but I'm not really familiar with Lemmy's front-end codebase to be able to do this myself, so I do what I can!