uups. I did not read your PS.
May be your are happier with this example, which defines a unique state variable for every macro call of the popup.
\define flag-title()
$(languagePluginTitle)$/icon
\end
\define make-popup(a)
<$let state={{{ [<qualify "$:/state/popup/language">] [<__a__>] +[join[/]] }}} >
<<state>>
<span class="tc-popup-keep">
<$button popup=<<state>> tooltip={{$:/language/Buttons/Language/Hint}} aria-label={{$:/language/Buttons/Language/Caption}} class=<<tv-config-toolbar-class>> selectedClass="tc-selected">
<$list filter="[<tv-config-toolbar-icons>match[yes]]">
<span class="tc-image-button">
<$set name="languagePluginTitle" value={{$:/language}}>
<$image source=<<flag-title>>/>
</$set>
</span>
</$list>
<$list filter="[<tv-config-toolbar-text>match[yes]]">
<span class="tc-btn-text"><$text text={{$:/language/Buttons/Language/Caption}}/></span>
</$list>
</$button>
</span>
<$reveal state=<<state>> type="popup" position="below" animate="yes">
<div class="tc-drop-down">
{{$:/snippets/languageswitcher}}
</div>
</$reveal>
</$let>
\end
<<make-popup aa>> --- <<make-popup bb>>
Also be aware of the tc-popup-keep variable. See the docs.