"Ukradli všechno, co mohli."

Translation: They stole everything they could.

[dsarkarati]

"They stole everything that they could." is also correct. "that" can be included or omitted.

[kacenka9]

yes. We would prefer if you could give suggestions via "my translation should have been accepted' button. Specially for instances where you are sure. Makes it faster and easier for us.

[Clive_Alive]

I think "They stole everything they could" should not be accepted. Only "They stole everything that they could" is correct.

[VladaFu]

Why do you think so? "That" in relative clauses can normally be skipped.

[nueby]

You are wrong. "That" as a relative pronoun can be omitted when it serves as an object in its clause. Have a look here. If you'd like a source with more heft, google

Zero is used identically to that except that it is unacceptable where the relative pronoun is subject in its clause

The above comes from section 4.88 in "A Concise Grammar of Contemporary English" (available for example here). The same text should be helpful when it comes to dealing with the who/whom and terminal preposition issues that seem to interest you (section 13.8).