Seems something where @lucatume can help. He will work soon on it.

I believe that’s the only option here because these Userinfos are shared. There is no content per site…

Interesting! Do you mind to make a pull-request at https://github.com/lloc/Multisite-Language-Switcher/?

Mmm … that’s a bit out of the scope of this plugin. There is a hook where you could add custom code for creating the necessary output but you would have to write it on your own.

I’m not sure because I don’t know that theme good enough. You could use the widget if there is any area right from the menu where you could place it.

This option was removed in Version 1.2

https://github.com/lloc/Multisite-Language-Switcher/blob/master/Changelog.md#12-2018-02-01

because you can set the user language in your profile.

You probably get the error “function is already defined” or similar. Try this here (it gets in one function):

function my_msls_options_get_permalink( $url, $language ) {
	if ( 'es_ES' == $language ) {
		return str_replace( '/shop/', '/tienda/', $url );
	}
	elseif ( 'en_US' == $language ) {
		return str_replace( '/tienda/', '/shop/', $url );
	}
	
	return $url;
}
add_filter( 'msls_options_get_permalink', 'my_msls_options_get_permalink', 10, 2 );

I don’t speak french but I would suggest updating PHP to version 7 or higher at least.