Now I think I will manage to give a good explanation though.
Hello, EngTeach, and welcome to the Grammar Exchange.
I completely agree with Gustavo's answer. One good way to explain this, apart from focusing on the clause within which the relative pronoun is used ("who speaks the language"), is to relate the point to personal pronouns and case.
Pronouns have case, subjective or objective. Subjective-case pronouns (like he, she, we, I, they—and who) are used in subject position, and objective-case pronouns (like him, her, us, me, them—and whom) are used in object position.
To decide whether subjective-case who is needed or objective-case whom, we can form a sentence related to the relevant clause and substitute a personal pronoun. It will be obvious whether subjective or objective case is needed.
[relative pronoun] speaks the language
subjective case: He speaks the language.
objective case: *Him speaks the language.