polylang-all-languages.php

<?php
function valteri_pre_get_posts( $query ) {
	// Show only posts from 'blogi' category.
	if ( ! is_admin() && $query->is_main_query() && is_home() ) {
		// Get Finnish ID.
		$lang_fi = get_term_by( 'slug', 'fi', 'language' );
		$id_fi   = $lang_fi->term_id;

		// Get Swedish ID.
		$lang_sv = get_term_by( 'slug', 'sv', 'language' );
		$id_sv   = $lang_sv->term_id;

		// Show both languages.
		$tax_query = array(
			array(
				'taxonomy' => 'language',
				'field'    => 'term_taxonomy_id',
				'terms'    => array( $id_fi, $id_sv ),
			),
		);
		$query->set( 'tax_query', $tax_query );

		// For some reason we need this that also Swedish blog page shows both languages.
		$query->set( 'lang', 'fi, sv' );

		// Show only posts from these categories.
		$query->set( 'category__in', array( get_cat_ID( 'Blogi' ), get_cat_ID( 'Blog' ) ) );
	}
}
add_action( 'pre_get_posts', 'valteri_pre_get_posts' );