Comment 1 for bug 145886

Sorry, a correction is needed, under en_GB it doesn't show too. Some more extensive list of when it is present and when not:

Shows:
en_BW, en_HK, en_US, en_ZA
he_IL
hi_IN
zu_ZA

Does not show:
en_GB
es_ES, es_MX
fr_FR, fr_CA
it_IT, it_CH

Thunderbird doesn't seem to be in launchpad translations anywhere, so can't check if it is just a missing string. Users may be confused if applications behave differently depending on the locale.