1. C++ and Rust.
2. Wallets are typically open-source and cost nothing. A free wallet cannot be made into money. The Bitcoin miners, not you (the wallet creator), are the ones who cash in on the transaction fees.

Here is my opinion: Join a micro cap project that has some game changing technology and contribute to its success. This is the best way for developers to get rewarded (paid) in this market. Either you receive free coins or you invest very early in the project.  Then, you put effort into making the project successful. For the first time in years, I now do investing instead of coding. However, I am promoting my investments and looking for ways to put them on the right course as an investor using this strategy. In my situation, I assist with making business connections. Before investing your time, you simply need to conduct your own due diligence. Before investing in any coin, I conduct my own due diligence.