If an english string in a doc uses an entity that is not from global.entities, you *may* either:
1: define a localised version
English string says "This is an &english-word;"
your string would then say e.g. "Det är en &svensk-ord;", and you would have
&svensk-ord; defined in sv/user.entities
2: expand it in place:
English string says "This is an &english-word;"
your string would say e.g. "Det är en svensk-ord".
Either of these solutions are correct, which you choose is up to you, although i suggest a consistent policy is going to be a lot easier in the long run.