Translation:Matěj is my best friend, whom I tell everything.
Not good English. Better: Matej, whom I tell everything, is my best friend. That gets the relative clause next to the modified person (Matej) instead of "friend". It "sounds" better to native English speakers. ("Sounds better" is the reasoning my Czech tutors tell me for Czech constructions.) Even better is: I tell everything to Matej, my best friend.