Foreign guide question

Is copying an already existing guide owned by someone else and translating it acceptable, while having the author's permission / giving credit and leaving the copyright the same (if they are no longer an active member)?

Only if you have permission.

I'd say if you're crediting the author and have permission, it should be fine. It's no different than translating a book. The author still has their name attached, and the translator puts their name separately as a translator. As long as you're not misrepresenting the work as being your own, I don't think you'll have any problems.