Advancing quite a lot, but not being able to translate the sentences which explain the different status changes of a task:

Problem solved, had to add a:
case "es_ES"
on line 166 of PhutilTranslator.php. Otherwise it fails, and the catch Exception also fails as Language is not initialised.

Not managing to get it working. I see the locale, but when creating the PhabricatorSpanishSpainTranslation (I think you put Spanish instead of Spain by error), and renaming the class that way, translations aren't shown.

Amazing quickness with your comments. Will start translating most used sentences, and when I reach something useful for others I will upload it.