Translation:I am hungry.
To fully explain we need to break down the sentence to see what is literally being said:
[My, implied] おなか [belly, stomach] が すきました [became empty]。 すく means "to become empty".
In order to be hungry now, one's stomach must have already become empty. This is just how it is expressed.
Welcome to the wonderful world of homophones in Japanese. In this case the word does not even use a normal reading for the kanji, which is not that uncommon.
I cannot really comment on the etymology at present, but if you look in a dictionary it will show both お腹 and お中. Perhaps it is as simple as to beautify the word. Note however that お腹 is what comes up in when using IME — for me at least — by default when typing おなか.
"I am hungry inside" sounds like quite a strange mix of the two languages. You could certainly replace belly or stomach with inside in my example above, but it seems unusual at best to express being hungry inside in English.
They're etymologically related though. おなか originated as a term used by noblewomen in the Heian era as a sort of euphemism for the belly. So, it literally was one's "middle part", i.e., the middle of one's body. Nowadays it's probably best to just consider them homophones though
Every single language program I've used has taught a different way to say this and it's actually been a huge amount of work trying to remember which service requires which form as an answer.
onaka ga sukimashita
onaka ga suite imasu
onaka ga suita
Onaka ga akimashita
I assume there is a wide variety of formality levels here but does anyone know what the most commonly used/acceptable one is?
Well, the difference between sukimashita and suita is just polite vs plain. Duo defaults to polite, which is probably the best for beginners. Hetta also means the same thing as sukimashita/suita, just much less polite, likewise with hara and onaka. It's odd that it would be taught in a language program. Hara hetta especially is very informal. I'm not sure about harapeko or onaka ga akimashita. I've never heard of those two
Quoted from someone else:
お腹 (おなか) がすきました。Onaka ga suki mashita. “おなか/onaka” means belly; you are saying your stomach became empty. Casually, to your friends/family or someone younger, you could say: お腹 (おなか) すいた。Onaka suita. or even more casually, お腹 (おなか) ペコペコ！Onaka pekopeko! (I’m starving!)
When conjugating from plain present/future (dictionary) form「空く」, for polite present, negative or past tense the last う sound becomes an い; here turning the く into き and then adding either ます (polite present/future), ません (negative polite present future), ました (polite past), or ませんでした (polite negative past)
空く - suku - to empty (plain present/future)
空きます - sukimasu - to empty (polite present/future)
空きません - sukimasen - to not empty (polite negative)
空きました - sukimashita - emptied (polite past)
空きませんでした - sukimasendeshita - did not empty (polite negative past)
The sentence here おなかがすきました literally means "(my) stomach has emptied"