When individually creating a language selector for TranslatePress:
$arr = trp_custom_language_switcher();
$arr returns a multi-dimensional array with all the necessary parameters to render an individual language selector within a loop.
In the frontend, however, when the foreign language was activated, the entry of the array was deleted with the key
"current_page_url" => "https://example.test/"
In this case, the option "Force language in individual links" was switched on in Options under (Settings > Translatepress > General). After deactivation ("No" option), the field content is rendered correctly.