I saw it as well after inviting someone to test the app a week ago - their mobile picked the French locale for no discernible reason. They got on well enough, were stuck on an 'Elderberry' they wanted to add being missing, and I went to 'Settings' and changed it for them.

What it made me think, the language setting should be visible from the first interaction, in case it's wrong: on mobile it's one more reason to get a welcome screen (I've made #657 to track), and on desktop I think we should have the language setting in the top bar as on the live site.

the automatic language choice stuck via local storage

I also wonder if we should only be preserving the language choice when logged in "Remember me" mode?

Looked into this a bit:

if we should only be preserving the language choice when logged in "Remember me" mode?

wasn't a good idea - using sessionStorage is useless for language selection, because opening something in a new tab doesn't preserve the setting.

New proposed resolution is to only save the language when a user selects one through a dropdown, or upon logging in in 'remember me' mode, and to clear the setting on log out.

Something for the ideas basket: I've read it's possible to do a server side setting by IP - the server reads an IP, puts a tag into the HTML that the i18n language detector uses, and it's super-customised. Not sure how much I like it, following the browser setting seems more respectful of the user's intent!