Adding languages without setting language negoiation

Whilst trying to test a fields in core patch I was trying to set up another language.

I added the language on the Languages page (admin/config/regional/language).
Then I added a user and set their language to the new language and logged in as that user.
Then (as admin) I went to export the built-in interface (admin/config/regional/translate/export) but the resulting .pot file only contained entries for javascript translations and none for any modules.

After making sure my language domain was set up correctly I realised that I hadn't set up language negotiation correctly on admin/config/regional/language/configure

I think that it might be useful to warn the user if they have more than one language enabled that language negotiation should be set to a value other than default for users to actually be able to use the languages.

I've attached a patch that adds a warning to the status report (admin/reports/status) if there's more than one language and no language negotiation.

I think if the user adds more than one language then the warning probably ought to appear on the language list page too (admin/config/regional/language). Perhaps it should be requirements_error instead or requirements_warning - is there a use case for a site with more than one language but language negotiation set to none?