They both have the same meaning, but "because" is a formal way and "cause" is informal. In spoken English you may use cause but in written English it is better to use "because." I always use "because."

The common career that people who are professionalized in languages can pursue is, of course, teaching. But I think, you will have much more prosperous career in finance and economics if you incorporate your language skills.

The proper question is "What do you do when you are bored?". "What do you do when you are boring?" means to be boring by yourself. When I'm bored I start doing one of my hobbies: make different things with beads, knit bags, play chess or call my frineds and go out with them.

If you want to know English well then you should learn both. Learning lots of words is very important for learning any language however, you will start writing or speaking good English when you learn grammer. So knowing words but not being able to create sentances is normal for beegining. What comes to phrases, you can't always use them in all situations.