"I order cat food."
Translation:Saya pesan makanan kucing.
As always, the main noun comes first. The noun modifiers, adjectives, etc. come after it. In "cat food", "food" is the main noun, and "cat" is just modifying "food" making it mean that the food is for cats. If there is human food, there's cat food, there's also dog food.
"Kucing makanan" would be "food cat" and would mean that a cat, among others, is food. If there is food cat, there could also be food human, food dog, etc..
Maybe you mean "memesan" thats comes from the root 'me-pesan'..yes, it has the same meaning in this sentence, and should be correct.. anyway, if we try to order some foods in the restaurant, usually we just say to the waiter/waitress 'saya pesan dua ayam goreng dan satu jus apel' instead of using 'memesan'..
In this case, we need to go into more detail about prefix me-. However, you need to know first that bahasa indonesia has basic/base words that could be classified into one or more classes: noun, verbs, adjectives, etc... Here are the example of basic words that can be classified into verbs : Saya datang ==> I come Saya pergi ==> I go Saya ingat ==> i remember Saya lupa ==> i forgot
And most basic words need to add prefix me- to make it classified as verbs such as :
Me-lempar : throw Me(m)baca : read Me-pukul (p->m)/memukul : punch
Make verbs from tools: Me(m)-palu/memalu : hammer +ing Me(nge)-bor: drill+ing
So, in some cases it is not only about formal/informal but it's mandatory to add prefix me- in some contexts.