An extremely small outfit with extremely high selection criteria and a gruelling recruitment process, that's specifically structured to handle foreign recruits, and drills them extremely harshly in French language. Which offers a citizenship, that while nothing to sneeze at, is regularly not even taken up by some of its soldiers after they serve.
It's not a viable comparison to signing people up at the border to get US citizenship on any level.
However it could act as inspiration for any efforts to found a similar outfit in the States.
That's a really good point, although I think the smallness matters more in absolute terms, than relative terms. Both examples show that you can maintain a large foreign presence in your military though. The French Foreign Legion is highly, highly, selective though.
I don't know much about the Ukraine foreign legion, but being in an immediately existentially threatening conflict is a special case. I doubt the Ukrainians would wish for such a large foreign legion if they had the choice, although again, I don't know the history of its numbers prior to the war with Russia.
It's not a viable comparison to signing people up at the border to get US citizenship on any level.
However it could act as inspiration for any efforts to found a similar outfit in the States.