How do I migrate a multilingual site?
It turned out that some of the migration processes were stuck after the first attempt. Here is how I solved the problem and completed the migration of my multilingual site:


Download and enable the Migrate Tools module and Drush 8
Run drush help to verify that new migration commands such as migrate-status and migrate-import are enabled
Run drush migrate-status and check the migration processes; all statusses should be Idle, but in my case some were stuck on Importing
Use migrate-stop to stop any running processes; according to the documentation, this ''cleanly stops a running operation''
Use migrate-reset-status to set the status of stopped processes back to Idle; according to the documentation, this ''sets a migration status to Idle if it's gotten stuck''
Continue the upgrade via the web browser or Drush