The LANG environment variable is not set in any place that your Xcode can access. Perhaps because it was not installed in your build settings.
When executing the command below, all environment variables will be printed, LANG will not be among them.
xcodebuild -project Path/To/Your.xcodeproj -target "YourTarget" -showBuildSettings
Environment variables are managed in your schemes in the "Environment Variables" table, but I was not lucky to change them there. What I eventually did (since my headerdoc2html call was in the build script in my build phase of my target documentation), add this line to the top of my bash script:
export LANG=en_US.US-ASCII
After that, the message disappeared.
My general script, for the curious.
export LANG=en_US.US-ASCII /Applications/Xcode.app/Contents/Developer/usr/bin/headerdoc2html -o ~/Development/Path/To/My/Documentation ~/Development/Path/To/My/Project/Headers /Applications/Xcode.app/Contents/Developer/usr/bin/gatherheaderdoc ~/Development/Path/To/My/Documentation
(Xcode version 4.6.3)
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