"They cook traditional food."
Translation:De lagar traditionell mat.
Because you usually say "lagar mat" when you aren't specifying what you are actually cooking. I believe you cannot just use lagar on it's own, or it isn't correct Swedish.
Since you are specifying what you are cooking (traditional food), you don't have to have that "mat" directly after "lagar". If you said "jag lagar mat traditionell mat" you would be saying "I cook food traditional food", which i guess would be okay with a comma.
I am not a native, but this is how i learned the verb!
I hope that makes sense!