Gets the translation for the locale and library, caching teh result.
@param library
@param locale
@returns {Promise(translation)}
async function(library, locale) {
      var t = this;
      var id = locale + ":" + library.getNamespace();
      var translation = t.__translations[id];
      if (!translation) {
        translation = t.__translations[id] = new qx.tool.compiler.app.Translation(library, locale);
        await translation.checkRead();
      } 
      return translation;
    }