What do you call the place/system where soldiers eat outside, with usually a few food carts with huge pots, and soldiers standing in line to receive food in their mess kit and eat it?
In French: cuisine de campagne, popote
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The general term for that, is field kitchen. There is an article about them, here https://en.wikipedia.org/wiki/Field_kitchen . It also has some other terminology for referring to them.
The term you're looking for very much depends on where it would appear.
"Field kitchen" would be widely understood but is quite formal. For example, you might read in a military history book that "many of the German field kitchens were destroyed in air attacks".
Conversely, terms like "chow line" and "chow hall" are slang terms which are more likely to be used by the soldiers themselves, but these terms are Americanisms and not used in some other nations (e.g. the British forces).
Context is everything.
What you are describing is a soup kitchen (even if it does not serve soup).
Soup Kitchen n.
(Social Welfare) a place or mobile stall where food and drink, esp soup, is served to destitute people
(Military) Military a mobile kitchen
Note however that when the food delivery is not mobile (for example, food provided on a military base, even if the base is not permanant), a term is a mess hall, especially if the food is served inside.