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That's a great question!

Fluent relies on Unicode Plural Rules [0] which allow us to handle all (as far as Unicode knows) pluralization rules for cardinal and ordinal (and range) categories :)

[0] http://cldr.unicode.org/index/cldr-spec/plural-rules

It's up to the localizer to define variants corresponding to the language's plural categories. For Russian, that's (one, few, many). Interestingly, this particular example could simplified to (few, *), because "раз" is good for both 1, 5, 11, 55, etc. See https://projectfluent.org/play/?id=7d22f87c04b23b86d9f9149d5... for an example of this in action.

Authoring tools can help here, too. Pontoon, Mozilla's translation management system, pre-populates plural variants based on the number of plural categories defined in Unicode's CLDR.

Arabic is even more complicated!

https://en.wikipedia.org/wiki/Arabic_grammar#Cardinal_numera...