is it required plugin to have permission system?
"Uses permission system (if applicable)"
I dont get it what means "(if applicable)" can I upload plugin without permission system implemented in it?
also
"Language API usage (if applicable)"
I'm sorry I dont really get it, are there any necessary things that should be implemented in myplugin.cs before uploading it on umod to approve?

You don't have to use permissions, but most will ask for that so it is encouraged.

There are multiple guidelines for acceptance, most of which can be seen when submitting. The messages using localization is a requirement.