After language change should be "Exit now?", not "Restart now?"
When you change the language of Spark interface it warns you that changes will take effect ony after Spark restart. And then it asks "Restart now?". But if you press Ok, it will only exit. So it should be "Exit now?".
This patch now allows the language plugin to restart spark instead of the exit.
The patch also allows plugins to have the ability to restart Spark if needed.
Looks like an easy fix, push this into 2.5.9