Theseus and Leta were shown to be operating out of the French Ministry in an earlier scene. Their boss, Travers, shows up a few scenes later and starts ordering them around.
INT. MINISTÈRE DES AFFAIRES MAGIQUES, MAIN LEVEL—NIGHT
THESEUS exits a meeting room and strides toward LETA, who is waiting for him.
LETA: What’s happening?
THESEUS: Grindelwald’s rallying. We don’t know where, but we think it’s tonight.
I think it's fair to say that given the large number of aurors (50+) that show up later in the film that this was a multi-jurisdictional taskforce operating within France but under the general authority of what Kama refers to as the ...
"Ministries of Europe"
... for the purposes of defeating a Europe-wide threat. Grindelwald represents an existential threat to the whole wizarding world and is alleged to have committed crimes across the continent, largely ignoring borders and boundaries. That being that case, it would seem wise for Aurors from a range of countries to work together.
SPIELMAN: Six months are enough. It’s time for him to answer for his crimes in Europe.