SemanticMerge has a commandline option for this purpose, --nolangwarn
. For example, in my Git configuration file I've configured SemanticMerge for diffing like so:
[difftool "semanticdiff"]
cmd = C:/Users/Arve/AppData/Local/PlasticSCM4/semanticmerge/semanticmergetool.exe -d \"$REMOTE\" -s \"$LOCAL\" --nolangwarn