Validate parameters before calling API

[hedyhli]

For example if the user provides a language 'does not exist', the response will be an empty list.

In this case we should check the language against the list of available languages (#4) and raise a ValueError saying the language is not correct.

Same thing for since and spoken languages. (The only three possible values for since is daily, weekly or monthly)

For the list of languages and spoken languages see #4

Please also add tests (pytest raises) to check invalid arguments