title: How to add <select> to theme header from plugin?
body: <p>How to add drop down option (<code>&lt;select&gt;</code>) in theme header (frontend). Just like the option to select language among available languages on sites with multi-language support.</p>
  
  <p>But instead of making changes in theme, is it possible to add it from plugin through some hook?</p>
  
  <p>I have in mind to add a function, and register its shortcode to be added in theme, but is there any better way to do this?</p>