Successful DPL queries stop $parser->getOptions()->getUserLang() from working for the rest of the page but only in some namespaces (?)
But if we repeat this in user namespace there's no issues at all, everything works as intended.
Template behaves the same way as Archive.
If you change language and save, the language you last saved in is what will be cached and show regardless of what you change it to, unless you re-blank-edit with the new language.
Archive isn't a FR namespace so that's probably not doing anything.
I'm pretty confused about this especially because it seems to be only some ns and I can't figure out what the difference is but there's definitely something weird happening.