[ithkuil]

In Croatian (and possibly other related languages) the number two (dva), like one (jedan), declines according to gender and case

Jedan prozor, jedna jabuka Dva prozora, dvije jabuke

Another unrelated quirk in Croatian (and I don't which other related language) is that with numbers 2, 3, 4 you use the genitive singular and with higher numbers the genitive plural

Tri prozora, tri jabuke, tri kune Četri prozora, četri jabuka, četri kune Pet prozora, pet jabuka, pet kuna (notice the -a ending)

I learned that when I wrote some software that converted numeric values in textual form. I never noticed the rule despite speaking correctly (born in a croatian family but I didn't attend school there so I only picked up the language by listening family members, reading tv subtitles and books).

I wonder if an average Croatian is aware of that plural forming rule or they just do the right thing without noticing.

[nihzm]

Slavic languages in general seem to have retained this case and gender dependent declination of numbers, making them rather difficult to learn. For example most languages on this list [1] have complex declension tables for the number two.

https://en.wiktionary.org/wiki/%D0%B4%D0%B2%D0%B0

[ithkuil]

But it only works for numbers 1 and 2 and goes away for bigger numbers.

Probably a remnant of the dual case of Proto Indo European