Installing the extension twice didn't seem to work, so I've used a restored profile which uses the original en_US in names of the path and contains translated files in the directories of CALENDER.DTD and LIGHTNING.DTD files. So, without a modified XPI file and without having removed the UI language en_US:
1. Download and install Lightning/2, including a restart of SM/TB
2. Download and unzip a matching, translated Win32 release (*DE* instead of *en_US*, for example)
3. Go to the directory of Win32's CALENDAR.DTD file
4. ZIP -r CA.ZIP * (LZH, for example, may or will fail)
5. Go to the directory of Win32's LIGHTNING.DTD file
6. ZIP -r LI.ZIP *
7. UNZIP CA.ZIP in the directory (long, mentioned above) where OS/2's CALENDER.DTD file is installed, overwriting files
8. UNZIP LI.ZIP in the directory (long, mentioned above) where OS/2's LIGHTNING.DTD file is installed, overwriting files
I guess en_US is no problem as long as it matches the file CHROME.MANIFEST, and/or it doesn't know that "Schadenfreude" actually is DE instead of en_US. If you haven't installed Lightning yet, then creating a new OS/2 XPI, mentioned earlier in this thread, is probably a better option. Do not forget to create a profile backup. You may need it.
The original functionality isn't that clear, but it allows SM/TB to e.g. understand Microsoft Lookout appointments, and you probably can download an internet's ICS file which could contain ArcaOS-related events, and so on.