Modify gettext to look through plural translations

[tomasr8]

Closes #5073

This is kind of a hacky solution so I'm open to suggestions..

Basically, the problem is that if there is a plural translation ,e.g.,

# Polish
msgid "Convener"
msgid_plural "Conveners"
msgstr[0] "Lider"

the only way to translate Convener to Lider is to use ngettext(..., 1). gettext ignores
the plural translations when looking for a translation.
The solution is to use our own gettext with a fallback to ngettext if a translation is not found.

I don't know if this 100% foolproof as there might be some languages where a normal translation
differs from its singular translation when used with numbers.

[ThiefMaster]

I think having a unit test for this would be useful.