New language keys are not available during upgrades

[mrclay]

If you need a translation you must call register_translations() on the main language directory before using the new key.

What would be the harm in just calling this before executing any upgrades?

Is this something the new translation system would fix?

[jdalsem]

This is still a problem, but would you accept the easy fix to add a cache invalidation to the "first" upgrade page step (upgrade.php line 63)? Because that will fix your problem.

elgg_invalidate_simplecache();
echo elgg_view_page(elgg_echo('upgrading'), '', 'upgrade');

That will make sure that the (language) cache is invalidated before the upgrade is commencing. The only minor issue is that it will invalidate the cache twice during this process.