As of Patch 4.0.1, released on October 12, 2010 this add-on seems to be no longer needed. For full details see the History section below. The add-on is kept for historical purposes; and in case the need for it arises again.
With patch 3.3.3 Blizzard's German localization team decided to change the way players' remaining buff and debuff times are displayed. They used to do it with one-letter acronyms like d for day and m for minute. The patch changed that to use Tag and Min. instead. This decision is questionable. Three letters don't fit up there and different buff times overlap. It's a mess. BuffTimeDisplayFixer reverts the localization team's changes back to pre-patch 3.3.3.
There is none. Install the add-on. From now on when you log in to the game BuffTimeDisplayFixer will reassign two constants the game uses for your remaining buff and debuff time display. The two constants that have changed are MINUTE_ONELETTER_ABBR and DAY_ONELETTER_ABBR. Therefore BuffTimeDisplayFixer consists of two lines. That's all there is to it.
Bugs and Suggestions
Please post bugs and suggest improvements at the ticket tracker. In case of new features, however, please note that I'm not particularly inclined to add unnecessary bloat to the add-on. There are currently no new features planned. With two lines of code I don't even foresee bug reports but you never know.
Consider BuffTimeDisplayFixer done. However, while it is unlikely, the add-on may very well melt your computer or show other unwanted behavior.
You can always find the latest alpha versions at the BuffTimeDisplayFixer CurseForge home page where I maintain the add-on. Alpha versions may melt your computer even faster than beta and stable versions.
In case the German localization team changes MINUTE_ONELETTER_ABBR and DAY_ONELETTER_ABBR back to their former shorter versions BuffTimeDisplayFixer will become redundant. On the other hand, it they go on with this behavior and change more strings BuffTimeDisplayFixer will be here to deal with it. In that case I might add additional strings to the add-on.
Coming up Next
Between March and October 2010 (starting with patch 3.3.3 and ending with the last day of patch 3.3.5a) German World of Warcraft clients used to show remaining buff and debuff times with strings that didn't quite fit underneath their corresponging icons. The buff and debuff bar looked unfinished and cluttered. Apparently this design decision from March 2010 was revisited for patch 4.0.1. Since then buff times look the same as they did pre-patch 3.3.3 and all is well and good once again.