Add language dropdown menu to My Preferences

[AntonKhorev]

Implements #2581 in https://www.openstreetmap.org/preferences/edit

[tomhughes]

There is some overlap here with #3618 which attempted to add a locale selector to the front page, only in as much as that if we did both we'd want to use the same selector in both places.

I've just rebased that on the current code (there's a live version at https://locale.apis.dev.openstreetmap.org/) but the styling of the selector is broken - think it's some conflict between bootstrap and the wikimedia styling of the language selector 😞

[tomhughes]

Is adding new languages at the start the best choice? Won't most users add languages starting with their first choice and then moving to other languages they understand?

[AntonKhorev]

If a language is added at the end, a user may choose the language, press the Add button and expect the ui to switch to that language. But it's probably not going to happen because another language that was in the preferences before is also supported and still has higher priority.

[AntonKhorev]

Won't most users add languages starting with their first choice and then moving to other languages they understand?

Maybe, but I expect most users to pick only one language. If they want to pick several, they are probably used to editing strings with multiple language codes and they are not going to use the dropdown at all.

[AntonKhorev]

Maybe change the button label from "Add" to "Prepend"?