Adds a fieldd to smart search and adds a filter to wp graphql where input and adds a meta query to the WP Query object

add-filters.php

<?php

add_action(
	'graphql_register_types',
	function() {
		$types = array( 'RootQueryToPostConnectionWhereArgs', 'RootQueryToPageConnectionWhereArgs', 'RootQueryToCustomPostTypeConnectionWhereArgs' ); // Add other types as needed.

		foreach ( $types as $type ) {
			register_graphql_field(
				$type,
				'languageThisPageIs',
				array(
					'type'        => 'String',
					'description' => __( 'Filter posts by language', 'textdomain' ),
				)
			);
		}
	}
);

add_filter(
	'graphql_post_object_connection_query_args',
	function( $query_args, $source, $input, $context, $info ) {
		if ( ! empty( $input['where']['languageThisPageIs'] ) ) {
			$language_value = sanitize_text_field( $input['where']['languageThisPageIs'] );

			$meta_query = array(
				'relation' => 'AND',
				array(
					'key'     => 'languageSwap.language_this_page_is', // This is the path to the ACF field which can be found on the search config page.
					'value'   => $language_value,
					'compare' => '=',
				),
			);

			if ( isset( $query_args['meta_query'] ) ) {
				$query_args['meta_query'][] = $meta_query;
			} else {
				$query_args['meta_query'] = $meta_query;
			}
		}

		return $query_args;
	},
	10,
	5
);