Where does “спонтанный” come from?

спонтанный (Russian) comes from German spontan, from Latin spontāneus, from Latin -āneus, from Latin -eus, from Proto-Italic *-eos.

Ancestry of “спонтанный”, step by step

Step	Language	Word	Meaning
1	German	spontan	spontaneous
2	Latin	spontāneus	—
3	Latin	-āneus	—
4	Latin	-eus	argentum + API → argenteus; ferrum + API →...
5	Proto-Italic	*-eos	—

спонтанный — Etymology, Origin & Word History | Etymology Explorer