Learn Spanish (Latin America): Los delegados deben ratificar el tratado, el cual es difícil. - The delegates must ratify the treaty, which is difficult.

You could absolutely use que here! However, el cual is a more formal relative pronoun, often used in writing, legal texts, or formal speech (which fits a sentence about delegates and treaties). Unlike que, it also changes to match the gender and number of the noun it refers to.

How do I know whether to use el cual, la cual, los cuales, or las cuales?

You match it to the noun it refers back to (the antecedent). In this sentence, "which" refers to el tratado (the treaty). Since el tratado is masculine and singular, we use el cual. If the sentence were about las leyes (the laws), you would use las cuales.

Can I use el cual without the comma if I want to specify exactly which treaty I mean?

No. When used without a preposition, el cual and its variations can only be used after a comma to add extra, non-essential information. If you want to define exactly which treaty you are talking about (e.g., "The treaty that we signed is difficult"), you must use que without a comma: El tratado que firmamos...

Could I say tienen que ratificar instead of deben ratificar?