[mikhayahu]

> double numbers, in addition to singular and plural (one hen, two hens, ten hens will be "[one] kura", "[two] kury", "[ten] kur")

Actually Polish does not have dual number, just singular and plural. What you are refering to is just different noun case - use plural genitive when numeral ends in 5,6,7,8,9 or multiple of 10.

[zoomablemind]

>...different noun case - use plural genitive when numeral ends in 5,6,7,8,9 or multiple of 10.

Exactly. gettext PO-file translation comes to mind, the plural form rules (e.g. https://php-gettext.github.io/Languages/ )

[rich_sasha]

Yes but in practice it’s the same for a learner (or only a small discount)