UPDATE! This is now a proper extension https://github.com/valu-digital/wp-graphql-polylang

graphql-polylang.php

<?php

/**
 * Add dummy query var which causes all languages to be shown
 * in the queries.
 * See https://github.com/polylang/polylang/blob/2ed446f92955cc2c952b944280fce3c18319bd85/include/query.php#L125-L134
 */
add_filter( 'pll_filter_query_excluded_query_vars', function( $excludes ) {
	$excludes[] = 'allLanguages';
	return $excludes;
}, 3, 10);


add_filter( 'graphql_input_fields', function( $fields ) {
  
	$fields['lang'] = [
		'type' => \WPGraphQL\Types::string(),
		'description' => 'Select posts by Polylang language',
	];

	$fields['allLanguages'] = [
		'type' => \WPGraphQL\Types::boolean(),
		'description' => 'Show posts regardless of the Polylang language',
	];

	return $fields;
} );

Usage