start point:
- Magento 2.2.6 set in developer mode
- Theme : Luma
- Cache: enabled
- 4 Store view enabled (one for each language):
- it
- en
- fr
- es
the only operation I did is migration from magento 1.9.1
what have I done?
Via ssh (in root folder of Magento) I ran the following commands:
composer require mageplaza/magento-2-italian-language-pack:dev-master
php bin/magento setup:static-content:deploy -f it_IT
php bin/magento setup:upgrade
php bin/magento cache:clean
php bin/magento cache:flush
( mageplaza takes the translation file from Crowdin (translated up to 93%))
set for Backend:
Click on user (on top right) > Account Setting (User) > Interface Locale > Italiano (Italia) / Italiano (Italia)

for frontend:
Stores > Configuration > Store view: Italiano > General > Locale Options : Italiano
Clean cache, Flush cache.
Open browser (with cache cleaned) and the results that I expect are:
- Magento with Luma theme translated in Italian : but only very few parts have been translated
- Backend of Magento translated (but only very few parts have been translated)

I can't understand why it does not work. Has it ever happened to anyone?