po file plural generation fix

Before the generation was:
```
msgid "%d unicorn"
msgid_plural "%d unicorns"
msgstr ""
```
but should be:
generation ->
```
msgid "%d unicorn"
msgid_plural "%d unicorns"
msgstr[0] ""
```
translated ->
```
msgid "%d unicorn"
msgid_plural "%d unicorns"
msgstr[0] "%d licorne"
msgstr[1] "%d licornes"
```