Hi Kenny,
There is two way to manage localization string.
1. Using design part -
<cms:LocalizedLabel ID="abc" runat="server" ResourceString="abc.sting"></cms:LocalizedLabel>
abc.string is resource string, you need to add that in localization.
2. Using Code -
SearchTextBox.Attributes.Add("placeholder", ResHelper.GetString("abc.string"));
SearchTextBox is ID of text box and placeholder is attribute you can use Text also and abc.string is localization string.