英语他们在做饭怎么说

In English, we would say "they are cooking" when referring to someone preparing food in the kitchen. Here are some other common phrases that might be used in this context:
They are making dinner.
They are preparing a meal.
They are whipping up something to eat.
It is also common to use more specific language to describe the specific type of food being prepared, such as "They are baking a cake," "They are grilling burgers," or "They are boiling pasta."