Where does “Алах” come from?

Алах (Serbo-Croatian) comes from Arabic اللّٰه, from Proto-Semitic ʾil- — deity; god.

Ancestry of “Алах”, step by step

Step	Language	Word	Meaning
1	Arabic	اللّٰه	God, Allah; the All-highest”, “God; the...
2	Proto-Semitic	ʾil-	deity; god