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Installation on non-US Windows fails since name of programs directory is different, e.g. "Programme" in German instead of "Program Files". I have no experience with VBA,. but that should be fixed easily by replacing the hardcoded "C:\Program Files" with a reference to % APPDATA%...
The problem with using %APPDATA% is that the formulas will then include the username in the path to the add-in, so if somebody else gets your sheet that person will need to edit all formulas. We could try %ProgramFiles% instead, but then sharing between different locales will result in the same problem as with %APPDATA% I guess. Maybe simply creating 'Program Files' if it doesn't exist is best or simply install in c:\sparklines as Fabrice first suggested when we discussed this problem.