After further investigation, it appears the list of missing locales was in-correct. Apparently the calendar all-locales/shipped-locales files are not really used for anything, and we should get rid of them. Checking released files, both from ATN and localized Thunderbird, I found they all have the same locales as Thunderbird does. Some will have more en-US strings than others.
Bug 1493008 Comment 21 Edit History
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After further investigation, it appears the list of missing locales was in-correct. Apparently the calendar all-locales/shipped-locales files are not really used for anything, and we should get rid of them. Checking released files, both from ATN and localized Thunderbird, I found they all have the same locales as Thunderbird. Some will have more en-US strings than others.