Same thing happens to me while trying to log in with language=auto
Workaround for me was to change the value to my local language like "lanuage=german"

The other workaround for now would be to copy your default language file to "strings_auto.txt", or if you're handy with the shell, to make that a symlink to your default language file.

cd /path/to/mantis/lang
ln -s strings_german.txt strings_auto.txt

Same thing also applies if you try to request a lost password.

$g_default_language can be set to 'auto', but $g_fallback_language cannot be (in case the browser language can't be figured out).

The checks for language not set, and 'auto' need to be reversed.

fixed in CVS, order of auto, null language checks reversed to allow $g_default_language to be 'auto'.