[Casiopea]

Originally Posted by Lenka

I'll try asking a Czech English teacher (Can I say it like this? I mean an English teacher whose mother language is Czech.).

Yes.

Note,

• 1. That
     a) For people and things and in conversation.
     b) After the following: all, everything, something, anything, nothing, none, little, few, much.
     c) After superlatives; e.g., the best.

Source: Relative Pronoun / ESL Lesson

Note also,

NOTE: It is preferable to use that (not which) after the following words: all, any(thing), every (thing), few, little, many, much, no(thing), none, some(thing), and after superlatives.

Source: Using Relative Clauses for ESL EFL TESOL TEFL TOEFL English Students Teachers and Classes - Upper Level and Advanced Level English Learning Skills