Does not protruding the tongue when pronouncing the word "الذين" (allatheena) in Surat Al-Fatihah invalidate the prayer, and if so, what should one do if the worshipper has not been protruding their tongue when pronouncing it previously?

The point of articulation for the letter "dhāl" (ذ) is between the tip of the tongue and the edges of the upper incisors. It does not require the tongue to protrude outside the mouth, but only to touch the upper incisors. If your pronunciation of "dhāl" conforms to this point of articulation, then it is correct. If it is otherwise, you must rectify it. There is no blame upon you for what has passed, as this does not invalidate the prayer.