Drobniutka pomoc ;)

Które zdanie jest poprawne?

We didn't leave this restaurant until the weather wasn't getting better.

czy może....

We didn't leave this resteurant until the weather was getting better.

???

No chyba że żadne z nich ale to już inna sprawa... Z gory dzieki za pomoc ;)

Try this:
We didn't leave the restaurant until the weather got better.
'Wasn't getting' not sound right in this context.
Please write the sentence in Polish, so I can see what the meaning is.

Nie opusciliśmy restauracji dopóki pogoda nie polepszyła się. W każdym razie dzieki za poprawienie błędu. Wykorzystam twoją wersję. ;)

Maybe this later version could be better:
We didn't leave the restaurant until the weather had got better. (English)
...the weather had gotten better (Amer)

Dlaczego had got?
IMHO We didn't leave the restaurant until weather got better.

powinien byc past perfect bo to byl niejako warunek ich wyjscia.

i dopieo po spelnieniu tego warunku czyli "dokonaniu sie" zmiany pogody oni wyszli

Yes, I agree with you that Past Perfect should be used.
But, as an auxiliary verb 'have' is the marker of the Perfect aspect.
Past perfect of have is 'had'.
The past tense of 'get' is 'got. The past perfect of get is 'had got'.
If you just use 'got' you would be using the past tense and not the past perfect tense.
This is long-winded but I hope that this explanation helps.

A nie powinno być
We hadn't left the restaurant umtil the weather got better
???

Yes. This is o.k. as well -
We had not left...