French is one of two co-equal official languages of the North Atlantic Treaty, and OTAN is just as much the official acronym of the organization as NATO is.
[0] https://en.wikipedia.org/wiki/2009_Strasbourg%E2%80%93Kehl_s...
Those acronyms in particular, SIDA and OTAN, are also the proper ones in Spanish and Portuguese.
It's not a translation, it's the official acronym based directly on the official name just as much as the English one is.
The Treaty and the Organization have two co-equal official languages, English and French.
> There are probably (?) more German-speaking NATO citizens
Probably, but the official languages, names, and acronyms were all set when the organization was founded, while Germany was still under occupation by the WWII Allies.
(Well, outside of CS, GMT is much more common than UTC here in the UK. But I'd put that more down to self-centricism and not having a different term for our own non-Summer time than translating the acronym (which of course it isn't, and came first).)
UTC isn't French. It's also not English. It's a deliberate compromise between the obvious acronym for the English phrase (CUT) and that for the corresponding French (TUC).
Though to me it reads more as English, and specifically American and military, because “Universal Time, Coordinated” follows a common pattern in US military naming, when what would normally be the first leading adjective instead becomes a trailing modifier.