Add support for language

[lucianghinda]

There are multiple resources (newsletters, blogs, courses and authors) writting in other languages than English.

I need to add support to tag each resource with language so that the reader knows what is the language used there.

The main language of the Ruby and Rails website will remain english, but near each resource we should add a field called language which should have a default en.

We should also add filters in the interface by language.