What software language do you use when developing plugins? 
Are you using C or C ++
What would you suggest to someone who is just starting to write the plugin.
What would you suggest to avoid making mistakes when you start this business?

Thank you.

The majority of the developers here are using C#.  C# is a high level language that has C-like syntax.

We are working on improving the documentation but most look at existing plugins to learn the basics.

The learning curve really depends on how familiar you are with C# and the particular game or game engine you are developing for.