Add support for `is-installed` to check if a given language is installed

[Dominik Schilling]

Core has it, themes and plugins have it, but languages not.

Doesn't seem like there's a specifc reason for not having it. We probably only need to copy parts of the install() method. Happy to give it a try if it gets accepted.

[Alain Schlesser]

@ocean90 Yes, that seems to be missing. I will gladly accept a PR for this.

[Dominik Schilling]

Submitted PR in #36.