format_plural() correction on singulars

I'm using the Czech locale. Trying the 6-dev version, translations like "1 item" are broken, showing "@count" to the public.

The plural model for our language (as well as others, Polish for example) is a bit complex, and involves (beside more plurals) also re-using of the singular form, which applies to every count with end-digit being 1, excepting 11. So for example "21 items" shows with singular correctly, called at the appropriate place of format_plural() code after evaluating the plural formula - of course this uses '@count' in Czech translation string for "1 item" ! But if the count is 1, it's called from different point in the code, where '@count' is not available yet. We need '@count' to be available even if English text is only just literally "1 item". It worked well in 5.x.

I'm attaching a patch, which simply just swaps two lines, to provide '@count' where necessary. I also noticed, that the "1 item" t() call missed the language code, as opposed to all the plural forms, so I added this also.