returns custom api with category *chanfe the Fa to any thing you want. i use this for multiple language site

endpoint that returns custom api with category

<?php
/**
 * Plugin Name:Ali Custom API 
 * Plugin URI:
 * Description: Crushing 
 * Version: 1.0
 * Author: Ali
 * Author URI: 
 */
 add_action( 'rest_api_init', 'register_custom_endpoints' );

function register_custom_endpoints() {
    register_rest_route( 'myplugin/v1', '/posts-by-category/', array(
        'methods' => 'GET',
        'callback' => 'get_posts_by_category',
    ) );
}

function get_posts_by_category( $request ) {
    $category_name = 'Fa';
    $category = get_category_by_slug( $category_name );
    $category_id = $category->term_id;

    $args = array(
        'category' => $category_id,
        'posts_per_page' => -1,
    );

    $posts = get_posts( $args );

    $data = array();

    foreach ( $posts as $post ) {
        $data[] = array(
            'id' => $post->ID,
            'title' => $post->post_title,
            'content' => $post->post_content,
        );
    }

    return new WP_REST_Response( $data, 200 );
}
?>

more information

add_action( 'rest_api_init', 'register_custom_endpoints' );

function register_custom_endpoints() {
register_rest_route( 'myplugin/v1', '/posts-by-category/', array(
'methods' => 'GET',
'callback' => 'get_posts_by_category',
) );
}

function get_posts_by_category( $request ) {
$category_name = 'En';
$category = get_category_by_slug( $category_name );
$category_id = $category->term_id;

$args = array(
    'category' => $category_id,
    'posts_per_page' => -1,
);

$posts = get_posts( $args );

$data = array();

foreach ( $posts as $post ) {
    $data[] = array(
        'id' => $post->ID,
        'title' => $post->post_title,
        'slug' => $post->post_name,
        'excerpt' => $post->post_excerpt,
        'featured_image' => get_the_post_thumbnail_url( $post->ID ),
        'content' => $post->post_content,
        'date' => $post->post_date,
    );
}

return new WP_REST_Response( $data, 200 );

}