Translation:Matěj is my best friend, to whom I tell everything.
The English translation needs the preposition "to". Matej is my best friend to whom I tell everything. OR Matej is my best friend whom I tell everything to.
The sentence to me implies that there are better friends to whom I'm not saying everything...
I lean towards the first comment. "To whom", while grammatically correct, would most likely not be used in spoken discourse anyway, colloquial American English is well on the way to forgetting about the who-whom distinction.
In which way do you lean towards the comment? I do not see any connection. There will be cases of more bookish and more colloquial sentences throughout the course. Be prepared for that. Both in English and in Czech. This course does NOT aim just towards spoken English or spoken Czech.
While that's true, a combination of colloquial (omitting "to") and formal (using "whom") is a bit like saying Je to dobrej velký člověk. Maybe someone would say it, but is it realistic to mix formal and colloquial?
"To whom" is also accepted, and has been since about the time that the issue was raised.
There is probably no harm also changing the main translation, if it will make it more natural or more consistent.