There is the Daichi's translation guidance into Xfce. There is a list of softwares to use for translating. And there is a list of languages and language codes to use for the PO filenames.
The list below are specialized softwares for PO files with support like message sorting, table view, fuzzy toggle button, statistics, PO header support, plural forms…
Text based editors with PO extension:
Mousepad, Leafpad, or any other notepad-like application will do but they aren't recommended.
Xfce is translated into many languages, as long as someone has taken the effort to create this translation. The translation status section show you exactly which languages are already covered and which ones still need work or are even missing. Please check out those webpages before joining to or creating the translators team for a specific language.
References: