Do what you know. There are many programming languages out there on the market. When you do freelancing, you should remember that clients pay you to solve problems. You are not paid to learn new tools and languages. It is always good to expand your programming skills to learn other languages. However, you should only bid for the job that you are confident with.
Note: There are dozens of other survey sites out there. These just happen to be the more well-known ones that have a reputation for paying out in a timely manner and not having impossible standards for cashing out. As always, be careful with your personal information. I recommend using an entirely separate email address for survey sites so they don’t clutter your inbox.