xfce-rc: Fix support for the LANGUAGE environment variable

When LANGUAGE contained more than one language code and there was a translation to the first language in a .desktop file, a translation to the second language was returned in the case when the line for the second language was above the line for the first one. Now translation for the first language is returned regardless of the line order. The entry != NULL condition is inverted to make the diff smaller and reduce the amount of indentation.