If Croatia and Serbia want to move to the Baltic I won't stand in their way. But they were last heard of in the Balkans. Anthony Quayle, the Colonel Brighton in "Lawrence of Arabia", served as an SOE agent in Albania. Of course working with guerrillas was probably easier than blending into a civilian population with a language plenty of Germans understood. A lot of spy movies are based on the public forgetting that people speak different languages in other countries. I can read French about as well as English, and communicate effectively, but I couldn't begin to pass as French. Even the British military intelligence operatives who worked under cover in Northern Ireland, including women, reckoned on only being able to stand a minute or two's casual conversation.
The story of Jean Moulin is worth reading.
https://en.wikipedia.org/wiki/Jean_Moulin
So are the blue plaques you find on walls all over Paris, to resistance members and spur of the moment insurgents who died liberating the city, without an English-speaking uniform in sight. I was twenty when I saw one on a street corner near the foreign ministry where the tank "Quimper" was destroyed when General Leclerc's Free French got in, and I had a ticket to Quimper for my summer course in my pocket at the time. I believe it is the picture in the top centre:
http://liberationparis70.paris.fr/fr/oeil-d-expert/
A British war correspondent who had seen a very great deal, said that the bravest thing he ever saw was General de Gaulle walking up the half-dark aisle of Notre Dame de Paris with people firing submachine-guns, and not knowing whether it was
feux de joie or something else. Being French they didn't damage the great rose-window, though.
I think it was WB Yeates that said "Unhappy the land that has need of heroes". But France did, and had them, as many as anywhere else could ever have.
I have a couple of very interesting little booklets. One is the British army's booklet for soldiers in France after the invasion, preparing them to meet a population impoverished, malnourished, culturally different, intensely nationalistic and often eager to be more generous than they could afford. The other is the American army's "112 Gripes about the French". Both are highly admirable books. But "112 Gripes" seems aimed primarily at smoothing over friction. The British version encourages men to actually enjoy the French, as if they were only twenty miles away and likely to be on friendlier terms than they used to be.
https://www.e-rcps.com/gripes/