"Matěj je můj nejlepší přítel, kterému říkám všechno."

Translation: Matěj is my best friend, whom I tell everything.

[dsarkarati]

"Matej is my best friend whom I tell everything to." is correct grammatically. "Matej is my best friend who I tell everything to." is colloquial English.

[dsarkarati]

The translation given ("Matej is my best friend, whom I tell everything.") is missing a necessary preposition: "to". Correct answers would be:
"Matej is my best friend whom I tell everything to." OR "Matej is my best friend to whom I tell everything."

[Franta540714]

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.

[VladaFu]

That makes it a different sentence. The original has therelative clause expandi g "přítel". Keep it that way in your translations.