How to add a language dropdown switcher to top panel?
I believe Flags does not integrate with Language switcher dropdown (contrib module) it only integrates with the core language switcher block

Here is some alternatives

1. Language Switcher Dropdown + languageicons


Install both modules 
Go to config &gt; regional and language &gt; language icons and choose  


  Icon placement
    Before link
    After link
    Replace link

Go to Structure &gt; block layout add Language dropdown switcher block to your desired region 


2. Bootstrap Languages

Compatible with bootstrap based themes only


Install module 
Go to Structure &gt; block layout add Bootstrap Language switcher block to your desired region 


3. dropdown_language

With this module icons have to be setup manually using css