I also had this problem, and it turned out that the easiest way to determine which file is potentially UTF-8 is if you have hundreds of files to use the command line call:
xcodebuild -exportLocalizations -localizationPath <dirpath> -project <projectname> [[-exportLanguage <targetlanguage>]]
This will give you a more useful error:
The input file /path/to/your/file/filename.m cannot be read in UTF-8 encoding
xcodebuild: error: /Applications/Xcode.app/Contents/Developer/usr/bin/extractLocStrings exited with status 1
Hope this helps others :)
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