This is very odd and might be a bug. Since there is no Japanese language pack for Thunderbird, we don't ship the Japanese translation of TorBirdy. We will change that in the next release by shipping placeholder strings instead.
Though I am still not sure why it defaults to Arabic (probably because it is the first locale in chrome.manifest?) Unsure.
Any update on this? Not that it will magically resolve itself (!) but maybe you found out the cause or have an update since I can't reproduce this issue.
I've also tried changing the locale to English (export LANG=en_US.UTF-8). This makes TorBirdy start correctly in English.
I think the problem might be that TorBirdy doesn't find a language pack for Japanese and then defaults to the first language it can find --- and like you said Arabic seems a good candidate for that, since it starts with an "A" :)
Of course, this is just a hunch; I haven't taken a look at the source code or anything.
I'm working on integrating Icedove and Torbirdy in Tails and I also ran into this problem. This can be reproduced in a Vietnamese locale, one of Tails' supported languages. It is almost certainly reproducible in other locales too.
In this case there is a Vietnamese language pack installed for Icedove. Icedove's interface is displayed in Vietnamese but Torbirdy is in Arabic.
Speaking from experience with a very similar bug in Tor Launcher, that was fixed a year ago, I very much believe the issue is that in chrome.manifest there is:
locale castironthunderbirdclub en chrome/locale/en/
but this should be "en-US" for us to get (US) English as the default locale. The XUL localization system does treat "en-US" as the hardcoded default, but only if it is (exactly) present in the list of locales in chrome.manifest, otherwise it'll just pick the first one, which tends to be ar = Arabic. So a correct line instead of the "en" one would be: