FIX: Use culture in converters where possible

src/ValueConverters/StringFormatConverter.cs

@@ -35,12 +35,12 @@ public object Convert(object[] values, Type targetType, object parameter, Cultur
                     // try to load SmartFormat Assembly
                     var asSmartFormat = Assembly.Load("SmartFormat");
                     var tt = asSmartFormat.GetType("SmartFormat.Smart");
-                    miFormat = tt.GetMethod("Format", BindingFlags.Static | BindingFlags.Public, null, new Type[] { typeof(string), typeof(object[]) }, null);
+                    miFormat = tt.GetMethod("Format", BindingFlags.Static | BindingFlags.Public, null, new Type[] { typeof(IFormatProvider), typeof(string), typeof(object[]) }, null);
                 }
                 catch
                 {
                     // fallback just take String.Format
-                    miFormat = typeof(string).GetMethod("Format", BindingFlags.Static | BindingFlags.Public, null, new Type[] { typeof(string), typeof(object[]) }, null);
+                    miFormat = typeof(string).GetMethod("Format", BindingFlags.Static | BindingFlags.Public, null, new Type[] { typeof(IFormatProvider), typeof(string), typeof(object[]) }, null);
                 }
             }
 
@@ -61,7 +61,7 @@ public object Convert(object[] values, Type targetType, object parameter, Cultur
                 return format;
 
             var args = values.Skip(1).ToArray();
-            return (string)miFormat.Invoke(null, new object[] { format, args });
+            return (string)miFormat.Invoke(null, new object[] { culture, format, args });
         }
 
         /// <inheritdoc/>

src/ValueConverters/ToLowerConverter.cs

@@ -30,7 +30,7 @@ public object Convert(object value, Type targetType, object parameter, CultureIn
         {
             if (value != null)
             {
-                return value.ToString().ToLower();
+                return value.ToString().ToLower(culture);
             }
 
             return null;

src/ValueConverters/ToUpperConverter.cs

@@ -30,7 +30,7 @@ public object Convert(object value, Type targetType, object parameter, CultureIn
         {
             if (value != null)
             {
-                return value.ToString().ToUpper();
+                return value.ToString().ToUpper(culture);
             }
 
             return null;