The lightweight plugin extends the default functionality and pulls out the language selection to the admin bar so you can easily switch between them.
Use the salc_languages filter to control which languages appear in the dropdown:
add_filter( 'salc_languages', function ( $languages ) {
$allowed = array( 'en_US', 'cs_CZ', 'de_DE' );
$languages['available'] = array_filter(
$languages['available'],
function ( $lang ) use ( $allowed ) {
return in_array( $lang['value'], $allowed, true );
}
);
return $languages;
} );