Not a bug
Some hosting providers, like Pantheon, have very strict security and follow the best recommended practices for deploying a WordPress sites.
These hosting providers secure their instances and allow write permissions by the web server to only the uploads folder.
If you are using such hosting provider, then the new method of translating with String Translation, will create issues for you – because the server itself and WPML String Translation will not be allowed to write in the folder /wp-content/languages/ and the folders in the folder tree beneath that.
To fix the issue and have everything working, you should re-define the folder for languages of WordPress (normally located in /wp-content/languages/ )
For Pantheon hosting, you can do that by adding this code to wp-config.php
define( 'WP_LANG_DIR', $_SERVER['HOME'] .'/files/languages' );
We suggest to add this code after:
/** Define appropriate location for default tmp directory on Pantheon */ define('WP_TEMP_DIR', $_SERVER['HOME'] .'/tmp');
You can try then to go to Dashboard > Updates and run the WordPress logic to get all language packs by pressing the button “Update Translations”
It was reported to us that it is possible that you also might need to create the /languages/ folder and /languages/wpml/ folders manually before everything works properly (if they were not created).