It wasn't a case of mistakenly pressing zero. What happened to me was, I had installed/uninstalled a bunch of stuff with Weidu v236, testing various versions of my own mod. I had chosen English as my language and it was using the English dialog.tlk.
Later, I simply deleted Weidu (I'm using OS X and keep a single copy of Mac Weidu, with various symlinks called "setup-modname" to install mods) ... deleted Weidu and replaced the file with the v236.04 beta. Installed/uninstalled a bunch of stuff, it all worked fine.
Then I once again deleted the file, and replaced it with v237, changing nothing else, and it started using the German dialog.tlk rather than the English one. I deleted weidu.conf, weidu.debug, weidu.log, and all symlinks and .log/.debug files associated with my mods, and upon running Weidu again it asked me for my language and I chose English.
Somehow replacing the file caused it to switch the language it was using. I don't know why, but a few people have had the same problem.