Maybe it is regional. But Ive heard mainboard used for at least 15 years or so in Germany.

Well before the whole shift in language thats going on.

No it's not, it's quite old and I would have thought technically minded people knew that word.

Often used for appliances like washing machine and TVs.

I have a tinkerer friend who call laptop motherboards 'mainboard' to differentiate them from desktop computers where you can actually plug graphic cards and Ram.

In Norway motherboard is called hovedkort. Hoved is etymologically linked to Norse hǫfuð or English head, but in this context the correct translation is main. Kort in this context means a card as derived from a chart or piece of cardboard or paper. While the term moderkort could be constructed in Norwegian, it simply does not exist. An alternative is kretskort which means circuit board, but it doesn't really relate well to a mainboard. In any case, when dealing with Norwegian or even German developers they might wrongly misconstrue their own word for mainboard, and thus contribute to the word spreading, perhaps even where it shouldn't.