G'day! I came across this blocker when upgrading a test site from 3.2.2+(Build: 20170324)  to 3.3 (20170515). 

If es_es language pack is installed, the Moodle upgrade may not proceed and admin can get stuck in a loop. See attached. Potential workarounds include: 

1) add "&lang=es" to the admin/index.php notification page.
2) change the admin user language to "es".
3) set "$CFG->skiplangupgrade = true;" in config.php

Related to previous bug reports: MDLSITE-2668 and MDL-43502