I would say 「かのじょは　にほんごが　あまり　わかりません」　or 「かのじょは　ほんのすこししか　にほんごが　はなせません」 If you want to emphasize the fact "she can understand" or "she can speak" even if it's a little bit, you could say, 「かのじょは　ちょっとだけ/すこしだけなら　にほんごが　わかります」 or 「かのじょは　ちょっとだけ/すこしだけなら　にほんごが　はなせます」 Basically 「ちょっと」 is a spoken language and 「すこし」 is more formal.

I'd put かのじょはにほんごをちょっとわかります, as in "She understands a little Japanese." If you wanted to say "She doesn't understand much Japanese", you could say かのじょはたくさんにほんごをわかりません。Chotto is a softener, sometimes translated as "a bit". You use it to lessen the degree of something, like in the first sentence above. There are other ways to use it, but that's the main way I've seen it used. On an unrelated note, the "wa" particle in Japanese is written with the "ha" character. No clue why, but that's how it is. :)