"making_sure_not_bot" not found in language "und" #1776

Describe the bug

this only seems to affect certain bots so not a high priority issue but we are seeing a low volume of unhandled exceptions with a stack trace in our logs that results in a server disconnected error on the other side. the error in the anubis logs is:

http: panic serving xxx.xxx.xxx.xxx:xxxxx: message "making_sure_not_bot" not found in language "und"

sure enough, the app logs show server disconnected errors associated with subrequests to our anubis deployment that include

Accept-Language: und;q=0.9

Steps to reproduce

1. set Accept-Language to some nonsense value like und
2. get challenged by Anubis

Expected behavior

localize to a default locale when Accept-Language is malformed or not present

Your operating system and its version.

N/A

Your browser and its version.

N/A

Additional context

No response