I am not sure if this sentence is correct:
The word, from which the Polish, Ukrainian and Czech verbs 'mówić', 'мовити', 'mluvit' descend, means 'to say'.
It is certainly a restrictive clause, but normally restrictive clauses are not set off with commas, and sense would be less clear if we remove the commas before and after the clause.
What should I change to get this sentence correct?
There is no need for a comma here but both are accurate grammatically:
The word from which the Polish, Ukrainian and Czech verbs 'mówić', 'мовити', 'mluvit' derive means 'to say'.
The word, from which the Polish, Ukrainian and Czech verbs 'mówić', 'мовити', 'mluvit' derive, means 'to say'.
The person from who I learned the information had disappeared.
If you make it restrictive by using commas, and you remove what is between them, the resulting sentence must make sense on its own. That said, I would not use commas.
You can write it as restrictive or not. And the word is derive here, not descend.
Your first option would be to remove commas, which would serve as a visual clue that you want this to have a restrictive meaning ("that" and "which" are sometimes interchangeable in creating a restrictive sense).
Because if you want to keep the "from [?] .... descends", you cannot change "from which" to "from that". "From that" wouldn't work here. If you use a preposition, you need "which".
As the link above says, in AmE, "which" for restrictive clauses would generally be avoided, though not be considered incorrect. To avoid it, what you could also do is rephrase the whole sentence to say,
"The word that is the ancestor of 'mówić', 'мовити', 'mluvit' means "to say" rather than "tongue".
