The solution was here: Localization - gathering text produces no output and fails - #8 by dacabad
It also created the line in the Gather.ini ‘ExcludePathFilters=Content/Localization/*’ → although I don’t really understand what it has to do, because in that folder there is not a single word to translate, nor the configuration or anything like that.
Instead of moving the StringTables, I have selected the folders where the words to translate are (mainly the folder where the Widgets are) in the ‘Include Path Wildcards’, and I have not added Content/*.