Hello.... I have downloaded a translation file for Spanish (source.csv) from Magento 2 translation site. Then I added the language pack into app/i18n/es_es folder. Language pack consists on composer.json, es_es.csv, language.xml and registration.php.
After all done, I have changed store locale to Spanish (Spain) but nothing happened. What can i do?
I have also issued: php bin/magento setup:upgrade, but it did not work.
A Magento 2 language package is just an extension so you should not "download" it manually but should install it through Composer.
For example: how to install an extension from a Git repository: https://mage2.pro/t/25
You should see this video https://www.youtube.com/watch?v=p5dVRnkuBQQ
This is a tutorial, How to install the language package in Magento 2 downloaded from Crowdin.