It means someone is using a trendy "buzzword" to try and sell something.
"Hacking" originally meant a kind of cutting or chopping, usually to make a big thing into smaller things, but the effect is also to change the shape of something. It specifically referred to the action, a sharp chop, not to the cutting.
Later on, it started to be used by computer programmers. A computer program is a long list of instructions. You might cut off some of those instructions, or "reshape" the list by adding new instructions. So "hacking" means modifying something that already exists. The purpose may be to change the way it works, usually to try and get a better result. One result may be eg illegal access to a system, perhaps to steal something or take control of it. Another result may be to make the program work more efficiently.
More recently, people have adopted the idea of hacking = newer more efficient way to do something.
For example, there is a TED.com video about hacking online dating. Someone created a method to find thousands of potential boyfriends very quickly, and evaluate them so she didn't waste time with the wrong ones. She used a different method to most people, based on the same system and was successful.
These days, everyone claims to be hacking something. There are videos online about how to fold t-shirts, which claim to be "life hacks". It's very tiresome.
Language hacking promises to be a quicker easier way to learn languages. Someone has a new system, or has changed the old system, and they're promoting it in order to make money off language-learners. I have no idea if the program they're offering actually works, but I doubt that it's genuinely a "hack". Language learning is hard work. There are definitely ways to make it more efficient, but I would be wary of anyone promising a cheap, quick and easy way.