Always a difficult issue, annexing territory with approval of the population.
Not difficult at all. First of all, how do you even know, there was an actual "approval of the population"? Residents of an occupied territory "voting" in support of the occupying power — is that really that convincing to you?
But even if a respectable proportion did sincerely want Russia to come over and "protect them". Imagine Mexico "reinforcing" Southern California and Arizona... Why? To protect the Spanish speaking people from the imminent threat of "Nazis" — like Donald Trump and Joe Arpaio... And claim "historical ownership" of the lands going back to Santa Anna.
Will you really have any difficulty deciding, whether it was an "invasion" and seek a different term? What if they promptly organize a referendum on whether to join Mexico — will you accept its results?
we can still talk to them and respect them on the international stage
No, we can't. Russia delenda est. The country — since long before Lenin — is a constant threat to liberty and peace world-wide with nothing especially valuable to justify it... There is definitely nothing to "respect" them for — not even their lousy weaponry.