diff -up plasma-desktop-5.4.0/kcms/formats/kcmformats.cpp.C_locale plasma-desktop-5.4.0/kcms/formats/kcmformats.cpp
--- plasma-desktop-5.4.0/kcms/formats/kcmformats.cpp.C_locale	2015-08-21 05:52:33.000000000 -0500
+++ plasma-desktop-5.4.0/kcms/formats/kcmformats.cpp	2015-09-09 08:34:56.109261012 -0500
@@ -115,7 +115,7 @@ void KCMFormats::addLocaleToCombo(QCombo
     // "it_IT")
     // TODO: Properly handle scripts (@foo)
     QString cvalue = locale.name();
-    if (!cvalue.contains('.')) { // explicitely add the encoding, otherwise Qt doesn't accept dead keys and garbles the output as well
+    if (!cvalue.contains('.') && (locale.language() != QLocale::C) ) { // explicitely add the encoding, otherwise Qt doesn't accept dead keys and garbles the output as well
         cvalue.append(QLatin1Char('.') + QTextCodec::codecForLocale()->name());
     }