[imp] allow to translate user plugin + es-ES language

This creates translation files and structure for the user plugin. This opens the plugin to be translated to any Joomla! language.

This also adds the spanish language (es-ES). Relevant strings remain unstranslated because the Latch website is not translated itslef. So "**Application ID**" remains that way to allow users to easily identify that key string in the documentation.

If this PR is accepted I'll add the same feature for the modules.