Where does the comma come from? Rather than replacing it, the better way would be to fix whatever is producing it. I would guess it's some conversion that is missing StringConverter.InvariantCulture in its parameters.

From what I could gather, it's due to the .ToString() when converting from floats/double to string.
It is possible to pass a NumberFormatInfo to the float.ToString(), the issue is that the code also uses object.ToString() and this function doesn't have an overload for NumberFormatInfo.
Alternatively it would be possible to check the type of the object and then cast it to a float/double, but the simple .Replace() seemed like the simplest fix.

I tried adding this check and it has the same result. It's just a lot more code.

// Original code
// var strVal = value.ToString().AppendZeroIfFloat(setting.SettingType);

// My change
var strVal = "";
if (setting.SettingType == typeof(float))
{
    strVal = ((float)value).ToString(nfi);
}
else if (setting.SettingType == typeof(double))
{
    strVal = ((double)value).ToString(nfi);
}
else if (setting.SettingType == typeof(decimal))
{
    strVal = ((decimal)value).ToString(nfi);
}
strVal = strVal.AppendZeroIfFloat(setting.SettingType);

I can update the PR if this solution is more favorable.

Just do .ToString(CultureInfo.InvariantCulture).

Just do .ToString(CultureInfo.InvariantCulture).

object.ToString() does not have any overload functions, meaning the example you are stating, does not exist.
I have found that Convert.ToString(rawValue, CultureInfo.InvariantCulture) does work.

The issue with that is in the function private void DrawUnknownField(SettingEntryBase setting, int rightColumnWidth) at the line var text = setting.ObjToStr(setting.Get()).AppendZeroIfFloat(setting.SettingType); the setting.ObjToStr(setting.Get()) is not possible to make InvariantCulture.
Arguably this shouldn't be an issue, as the function is defined by the creator of the setting, and they should therefore have handled this case.

I have updated the PR, is this an acceptable solution?