The one you can use anywhere without much hassle and which you are most familar with.

For me it is PHP.

No, I would either start with C or python based on their patience level.

Heck, even Go or PHP is a decent choice, if they are interested in website backends.

For frontend the only king is javascript.

Well the coorporations have always used/misused FOSS tech. Take Chromium and VSCode as an example.

This is exactly what is going to happen, you will not be able to separate proprietary crap from FOSS and it will be applied to fediverse in the same manner.

But there is a silver lining, all this will have a side-effect of normies knowing a lot about fediverse (for better or worse).