TB 68.1.2(en-US build) + Japanese Language Pack68.0buildid20190909201201 : works for me(except imap/local folder).
Bug 1575512 Comment 33 Edit History
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intl.multilingual.enabled set to true, install lang pack, options>Advanced>General>Language > select a lang & restart TB 68.1.2(en-US build) + Japanese Language Pack68.0buildid20190909201201 : works for me(except imap/local folder).