It's good to expand on a language if you already know the basics. I wouldn't use a dub though, as I can't stand mismatched lip syncing. I'd rather find a series or movie that was originally made in the language you want to train yourself in.

It is not ideal if you don't know the language though, because subtitles are very often not accurate. Often times, some words are left out and some phrases are simplified to make them faster to read. Without any basic knowledge, you're also likely to not pick up a significant amount of grammar. At least in my experience.