Which is better to learn in the USA,Japanese or French, which would be more useful and open more opportunities

Why pick? I'm learning both. But if I had to choose I'd pick Japanese even though it's way harder. I'm not so sure about opportunities, though. French would be useful because there are a lot of people that speak French and Japanese would be useful for business.

Whatever you decide, learning any foreign language has its benefits.

I think either one will be great to learn, but Japanese would be a better option. It can open many doors because not many people speak the language, and jobs, especially in the government, need people who speak Japanese. However, japanese will be much, MUCH harder to learn. It has a completely different alphabet than what english and french is.

French is not widely spoken in the US, and is mainly spoken in France and some parts of Canada and Africa. Japan is spoken in Japan, but that might be it.
If you're looking for a challenge, go for Japanese! With practice you will master it.
=)

Chinese is a tone language and it might be less difficult to be taught Russian. I consider Spanish might be a sensible option considering it is the 2d spoken language after English. French may be spoken in lots of areas.

did someone say most french words are based on american english???

It depends on what you think you might want to go into...Japan is huge in business, but French is actually the most spoken language in the business world. French is derived from Latin as is English which would make it much more simple to learn than Japanese because it is very different. You also have to learn an enitrely new alphabet and the weird pronunciation. French is difficult to pronounce, but greatly more easy than Japanese.

I think that learning French is better in USA since most french words are based on American English. As well as french is similar to Spanish and some parts of USA do speak Spanish... French is the second / third language (most spoken in the world).

However, if you're hoping to do business then you could try Japanese and negociate with the Japanese people. Remember though, that only Japan speak japanese whereas French is spoken in Africa, France etc