I haven't ever used the multi language so this is just what i expect it to do.

Unless you have recorded a no answer greeting it will play a system prompt which should be in whatever language you have set.

If the user has recorded a greeting then it will play that greeting, it won't translate that into a different language.

Even mailboxes are set by default to use the system language, you can manually set a different language if you want to, but then it won't follow the system.