No Stupid Questions

Latin was the market language of Rome, and commanders/generals would have issued orders and received reports written in Latin.

Most soldiers would have spoken it, including the local auxiliaries that were conscripted. (Or at least a pidgin version of it.)

Even if the conscripts would speak whatever amongst themselves, they’d have understood Latin. (It’s also very likely that foreigners brought into the province would pickup at least a pidgin version of the local language.)

To clarify, this would be like the French foreign legion not speaking French. (The do. Maybe not natively, but French language skills are necessary for conscription.)

The issue at hand is that the EU is not an empire, it’s an economic alliance of sovereign countries each with whatever language they happen to speak. For an empire, it’s easy to dictate things like “Latin is the official language, all business is conducted in Latin.”

Armies don't need complex phrases like "Last week did I see you go to the library?" And as such they've often used non-verbal communications in battle (especially because battle is fucking confusing) this has included drum signals, colored smoke, flags for different actions, and gestures.

When they are using verbal commands they're often a greatly simplified set of phrases - sometimes in a jargon that isn't even proper speech. It's rare to see ten syllable commands and you're much more likely to hear "company, right" than "Company of soldiers whom I command, please initiate a pinwheel rotation to the right around the rightmost file of your formation. Thank you kindly." So if Europe had to form an army inter lingually then "a derech" and "a isker" would probably end up being right and left respectively... that or they'd follow the EU trend of "When in doubt, default to French"... or they'd just signal their troops with different patterns of Eurohouse beats.

There's a Wikipedia section here about one that is not ancient, but relevant to your question about multilingual states: https://en.wikipedia.org/wiki/Austro-Hungarian_Army#Linguistics_and_translations

Don't Europeans know basic English? Uk, basic enough to exchange coordinates and stuff? I doubt u would need to have a large enough vocabulary for this.

The World wars for example were fought together in a time where English competency wasn't as high as it is today.