The PO translation file format and MO translation file format natively support pluralization, making it easy to translate pluralizable strings using tools such as Poedit.

The PoFileLoader and MoFileLoader support the native pluralization.

The PoFileDumper and MoFileDumper, however, do not support the native pluralization. It would be really nice if this could be implemented.

The PO format is:

msgid untranslated-string-singular
msgid_plural untranslated-string-plural
msgstr[0] translated-string-case-0
...
msgstr[N] translated-string-case-n

For example:

msgid "found %d fatal error"
msgid_plural "found %d fatal errors"
msgstr[0] "s'ha trobat %d error fatal"
msgstr[1] "s'han trobat %d errors fatals"

This works very well when the untranslated language only has two pluralization forms (singular and plural). The translated language can have more complex rules. I don't think the native pluralization can be used when the untranslated language has complex pluralization rules, or when using Explicit Interval Pluralization.

Note: It seems that the Qt format also support pluralization natively (numerusform). The QtFileDumper does not support this, and it doesn't seem that QtFileLoader does either.