Internationalization support

**Is your feature request related to a problem?**
Currently mentorship-system supports only English – all messages sent by server are in English. We may think everyone knows it but it's actually *only* about 2 billion people ([source](https://en.wikipedia.org/wiki/English-speaking_world)).

If we want to be as inclusive as possible, we need to think about supporting more languages

**Describe the solution you'd like**
My first thought is to add a URL query param to every endpoint, for example `lang=en` (English) or `lang=pl` (Polish). Then we would select messages from `messages.en.py` or `messages.pl.py` file, which would contain all translated messages.

**Describe alternatives you've considered**
We can continue to support only English.

**Additional context**
I think that adding everyone would be happy to contribute by adding their native language :)