Dennis Ploetner

I would suggest a slight change, because it would give you more flexibility and it is a bit more future-proof. You could use do_action when you want to print the output of MSLS in various parts of your templates. Let’s assume you just put this in your subheader:

<?php

$args = array(); // this could held some config
do_action( 'zwibasti_language_switcher_output', $args ); // this triggers an action

In the functions.php (or in a custom plugin) you could have something like this:

<?php

function zwibasti_msls_print( array $args ): void {
    if ( ! function_exists( 'the_msls' ) ) {
        return;
    }

    $tags array(
        'before_item'   => '<li>',
        'after_item'    => '</li>',
        'before_output' => '<ul class="your-css-class">,
        'after_output'  => '</ul>',
    );

    the_msls( wp_parse_args( $tags, $args ) );
}
add_action( 'zwibasti_language_switcher_output', 'zwibasti_msls_print' );

What is the advantage? You can do it without looking in all files whenever you decide to change something or even use another solution. Also, with the centralized functionality, you can decide the minimal setup.

Where did you add the code? To a functions.php file in your theme or a custom plugin?

I guess that your code has a small flaw. The expected input for that function is an integer not an array:

<?php

$post_id = get_queried_object_id(); // or any number of your choice
$msls_post = msls_get_post( $post_id );
print_r( $msls_post );

You should be able to see an icon whenever you write a post (or other type of content) like in my example here: https://imgur.com/a/3Ck7ZPe

The most important things are that you have at least 2 websites with different languages, and that you configured them in the settings for the Multisite Language Switcher: https://msls.co/

Can you confirm that this has been done?

With version 2.9.2, I did start to provide more API functions and less access to the internals of various objects/classes. The API function that you’re looking for is called msls_output in would be safe to use in future versions too:

/**
 * Gets the MslsOutput instance
 *
 * @return \lloc\Msls\MslsOutput
 */
function msls_output(): \lloc\Msls\MslsOutput {
    return \lloc\Msls\MslsOutput::create();
}

It looks like some JS is not working as expected. At least I see some warning in the console that lets me think that. Another possibility could be an incorrect redirect rule.

But those are things that are not coming from the plugin.

How can you conditionally check if a translation exists? How can you retrieve the URL of the translation of the current page in another language?

You can probably hook into msls_filter_string (read on here https://msls.co/developer-docs/hook-reference/) or use an API function like get_msls_permalink() (read on here https://msls.co/developer-docs/api-functions/).