Go down the stairs, turn left, and turn left again OR Go down the stairs and keep left.

Sztaki definitely works, and is the one that I use the most, but I would definitely suggest picking up a printed dictionary if you can. They are simply so much more reliable in the translations, and include more examples of use and variety. They're well worth the money.

There is a difference when you're talking about someone. For example: "I went to the bar despite what she said" - Says that she told you not to go to the bar, but you went anyway. "I went to the bar in spite of what she said" - Says that she told you not to go to the bar, and you went to get back at her. Otherwise, what Yusuf said is correct.

There is a bit of a difference. Advice is more personal, like what you might give to someone who is having relationship problems. A suggestion is more impersonal, for example what you might give to someone who needs help with a presentation. It's a really subtle difference though, and in most circumstances they're interchangeable. As for when to use advice as singular or plural, "advice" is both singular and plural, so you don't need to worry about it.

If you mean in the context of that day, it would be more appropriate to say "I took two baths today"

They both mean the same thing, but typically are used a little bit differently. For example: "The refugees were treated badly." "There was ill-treatment among the refugees." "The refugees were not treated well." All three are just different ways to say the same thing.

I don't think "in term of" is correct. At least, if it is, I've never heard it before.