Rated 4 out of 5 stars
Till the developer self will extend the localizations, you can simple set it up as following:
- download the add-on data (currently, "filter_copy-0.2-tb.xpi")
- unpack it (XPI = ZIP)
- open the directory [unpack_folder]/chrome/filter_copy/locale and find the folder "ja-JP"
- copy the folder ja-JP as en-US (for the English localization)
- edit the file en-US/overlay.dtd. This contains only one configuration entry. So, in place of the Japanese text, write: "Copy".
- go to [unpack_folder] and edit the file chrome.manifest: Add the new line (analog to the existing line with "ja-JP"):
locale filter_copy en-US chrome/filter_copy/locale/en-US/
(The same steps should be done for each other localization, e.g. ru-RU, de-DE, fr-FR... etc. One new folder containing a translated overlay.dtd + one new line in the manifest file for each localization.)
Pack again as ZIP, rename the "ZIP" extension into "XPI" and install into Thunderbird.