Which SEO plug to use?

Sorry, I cannot recommand you any seo plugin because I just didnt test them all… I think Yoast SEO plugin works fine with sublanguage, though its not really lightweight.

What is this seo plugin you’re talking about?

You can translate this plugin meta description and meta title by adding the following in your function.php file:

add_filter('sublanguage_register_postmeta_key', 'my_translate_postmeta');

function my_translate_postmeta($postmeta_keys) {

	$postmeta_keys[] = '_genesis_description';
	$postmeta_keys[] = '_genesis_title';

    return $postmeta_keys;

}

Unfortunately this won’t work when the description or title meta field is left empty: the meta description or title will always default to your main language post content or title. So you must make sure meta description and title are filled on every post, when not in main language.

Hello,

Extra-languages data of posts are stored in a custom post parented to this post. These children “translations” posts have also a specific post-type: “translation_{$id}”, where $id is a general id for this language.

This is a very basic snippet to interact with sublanguage.
You can find more info on github

global $sublanguage;
// or global $sublanguage_admin when in admin.

$some_post = get_post(123);
// this will be the "main language" post

$current_language_id = $sublanguage->current_language->ID;
// the current language id.
// should be $sublanguage_admin->current_language->ID; wen in admin.

$translation_post = $sublanguage->get_post_translation($some_post->ID, $current_language_id);
// this is the "sub language" post (if actually on a "sub" language).
// it stores translations in post_content, post_title, post_name and post_excerpt fields.

$post_meta = get_post_meta($some_post->ID, 'some_meta_key');
// this is automatically translated according to the current language if post meta was previously  registrated with 'sublanguage_register_postmeta_key' filter

Please ask me if you need more specific informations.

Thanks to you 🙂