I think invoice is generated from BE but as FE view.
Sometimes FE language (order) is different as BE language (a lot of admin's working on other language then FE - on own shops admins working in CZ language, but shops are in others).
Then there is conflict between language and invoices are in wrong language.
But is very easy load order language from DB to PHP and setup in view for print to PDF.
Yes, some people check this as bug, some as not prepared VM to multilanguage