That Finnish Mythology I mentioned, it is barely remembered folklore, about as relevant to modern Finns than ancient druids to the modern Irish. And if someone would mine that mythology for a fantasy game, I obviously wouldn't mind, I might be pleased. (And D&D has done this. Mielikki is an ancient Finnish forest goddess.) But if the stated rationale why it was OK to do so was that the the culture was a cadaver ripe for plundering then I might be somewhat less thrilled. I fully get that temporal distance in these things matters, someone's living traditions are more important than ancient history, but the difference also isn't that clear cut. And considering that Celtic culture and language has historically been targeted by systematic attempts to eliminate it, your proclamation came across as a tad insensitive.