Fiends, the last upgrade overwrote my site translation. The spanish translation I've made was overwrote by the po files provided by the upgrade so all the time consuming work was lost.
How to avoid this in the future upgrades ?
Thanks
Please make sure your translation is kept in the wp-content -> languages folder.
The theme translation files should be present in the wp-content -> language -> themes and plugins translation file should be present in the wp-content -> language -> plugins.
If not present then please move the translation files there and after that you need to rescan or sync the files using your translation plugin.
The files present in wp-content -> languages folder are safe and are not lost after updates.