[Muas]

...btw, is this mod compatible with Gastons More Nations Mod?

No. After install this mod, you'll loose any other custom countries/squadrons you had before.
Refer to the readme before install. You'll have to manually add the other countries/squadrons.
It's simple text edit - not very difficult.

Is this mod compatible for online play, or does it still cause the invisible & useless problem?

We've yet to test that, but I believe that at least if all players have this mod installed, no one will be invisible anymore.

I have extracted archive contents in folders samples\Speech\EN and samples\Speech\ER in IL-2 root folder, but has occurred nothing What have I made incorrectly?

You'll need just one speech pack installed on folder samples\Speech\ES.