I have never heard the term "weak sauce." "Whack" means something like crazy, weird, bad, and it would sound very weird for a foreigner to say. Even in the U.S., it's slang used mostly in the Black or hip-hop community.

The phrase "weak sauce" is the kind of slang that you probably shouldn't use until you have lived around natives for a long time and know intuitively when to use it. For example, if your friends use it a lot, you might get a feel for how to use it. But if you just said it without having an intuitive feel, it would probably sound strange to a native.

I would avoid using these particular slang terms! Maybe just use the word “weak” by itself if really want to express this opinion. IE. “The cinematography in that movie was very WEAK”! “Weak Sauce” comes across somewhat immature. You might hear 7th & 8th graders speaking this way.

I agree with the others. The single word LAME works pretty well. It just means not very good.