Use Locale.ROOT for case operations

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/str/StringBuiltins.java

@@ -776,7 +776,7 @@ static String upper(Object self,
 
         @TruffleBoundary
         private static String toUpperCase(String str) {
-            return UCharacter.toUpperCase(Locale.ENGLISH, str);
+            return UCharacter.toUpperCase(Locale.ROOT, str);
         }
     }
 
@@ -903,7 +903,7 @@ private static String toLowerCase(String self) {
         }
 
         private static String internalToLowerCase(String self) {
-            return UCharacter.toLowerCase(Locale.ENGLISH, self);
+            return UCharacter.toLowerCase(Locale.ROOT, self);
         }
     }
 
@@ -2035,7 +2035,7 @@ static String doGeneric(Object self,
 
         @TruffleBoundary
         private static String doTitle(String self) {
-            return UCharacter.toTitleCase(Locale.ENGLISH, self, null);
+            return UCharacter.toTitleCase(Locale.ROOT, self, null);
         }
     }