**Background**
Many users have their interface language set to their native language, but are perfectly comfortable writing in English. Or vice versa: They have the interface language set to English, but normally write in their native tongue. In either case, they may stumble across the Wishlist, and write a wish in a language different than the interface language. What happens is then we need to fix things after the fact, and that's only when we happen to even notice the page language is wrong. We were warned about this problem very early on by @tstarling. It came up again in T409992.
We need to allow users to specify which language they write their wish in from the get-go.
**Acceptance criteria**
* For all users, when creating a new wish or focus area, a dropdown to select the language should be the first form element. I suggest the label: "Language that this wish is written in".
* The dropdown should default to the having the user's interface language selected.
* When //editing// an existing wish or focus area, the same dropdown should be present – but only if the user has the `pagelang` user right.
** Bonus points: Changing the language in this case should also change the actual page language (right now it's a two-step process). I don't think this is important though because we won't have to correct the language very often if the user is able to select the correct one in the first place.
** Changing the value //should// however update the `baselang` parameter in the wikitext accordingly.