Today on TiSlack (https://ti-slack.slack.com/archives/C03CVQX2A/p1603729246286000) I finally found the problem that doesn't load my strings causing this warning:
The problem is that during compile I saw GRADLE write:
For every string in my strings.xml file.
The "default" GRADLE is searching for is the "en" folder in "i18n" insted I had only the "it".
After adding that folder everything is working fine.
I don't know if it fixable but maybe put an info inside the documentation.