Please note that plural forms for particular translation unit are accessed
with integer index (and not string like 'zero', 'one', 'many' etc). This is
because they are indexed such way in XLIFF files in order to not break tools'
support.
There are very few XLIFF editors, but they are nice Gettext's .po editors
available. Gettext supports plural forms, but it indexes them using integer
numbers. Leaving it this way in .xlf files, makes it possible to easily convert
them to .po (e.g. using xliff2po from Translation Toolkit), edit with Poedit,
and convert back to .xlf without any information loss (using po2xliff).