Nothing is easier than this

I’d say there are even easier plugins for localization than this, but the ones I know are not well integrated with the standard support for localization of gettext files, but they save the strings localized in the database, so is a PITA exporting them to another sites, even if they seem easier to use at first.

So it’s not really the easier at first sight, but is a lot better than other solutions since you can take your work and put to other sites your plugins and theme translations.