I post in three languages on this site: English (mostly), Dutch, German (sometimes).
To better allow language detection I have added mark-up for it in various places. I also add translation links to the RSS feed items that are not in English.

Note: All my non-English posts are in specific categories that provide the language (e.g. nederlands for Dutch, and deutsch for German)

Adaptations made:

Translation links in RSS feed items

In functions.php of my (child)-theme I added (based on this code by Jan Boddez):

add_filter( 'the_content_feed', 'my_content_feed' );
function my_content_feed( $content ) {
	global $post;
$testurl = get_permalink($post);
if ( has_category( 'nederlands', $post->ID)) {
     $basetranslate = 'https://translate.google.com/translate?sl=nl&tl=en&u=';
     $testedurl = urlencode($testurl);
     $basetranslate = $basetranslate.$testedurl;
     $addedlink= '<br/><a href="'.$basetranslate.'">machine translation into English</a><br/>';
     $content = $content.$addedlink;
    }
    
if ( has_category( 'deutsch', $post->ID)) {
     $basetranslate = 'https://translate.google.com/translate?sl=de&tl=en&u=';
     $testedurl = urlencode($testurl);
     $basetranslate = $basetranslate.$testedurl;
     $addedlink= '<br/><a href="'.$basetranslate.'">machine translation into English</a><br/>';
     $content = $content.$addedlink;
    }


	return $content;
}

Setting page wide default language

The function for setting a page wide default considers two cases: when displaying a single post, and when displaying a category archive.
In functions.php of my (child)-theme I added:

function tonsempress_language_attributes($langtoets){
  // ton: if called from single or cat will have right language otherwise default
$langtoets ="en-us";
if (is_single()){
if ( in_category('nederlands') ) {
$langtoets = "nl-nl";}
if ( in_category('deutsch') ) {
$langtoets = "de-de";}
}
if (is_category('nederlands')){$langtoets = "nl-nl";}
if (is_category('deutsch')){$langtoets = "de-de";}
  // return the new language attribute
  return 'lang="'.$langtoets.'"';
}

add_filter('language_attributes', 'tonsempress_language_attributes');

Setting item language on front page

Items on the front page (!) that are in Dutch or German will get a span with their language attribute set accordingly. I changed this in index.php of the child theme, where it considers if a Dutch or German category post is being displayed, in the WordPress Loop:

<?php /* Start the Loop */ ?>
<?php while ( have_posts() ) : the_post(); ?>
                    
<?php
	/* Include the Post-Format-specific template for the content.
	* If you want to overload this in a child theme then include a file
	* called content-___.php (where ___ is the Post Format name) and that will be used instead.
	*/
?>	 
<?php /* if in Dutch or German categories, on the homepage added Oct 2019*/?>
	<?php  if ( is_home() ) {
                    if ( in_category( 'nederlands' ) ) { ?><span lang="nl-nl"><?php }
                    if ( in_category( 'deutsch' ) ) { ?><span lang="de-de"><?php }
                    }
						 
                    
	get_template_part( 'content', get_post_format() );

	/* if in Dutch or German categories, on the homepage added Oct 2019*/
		if ( is_home() ) {
                     if ( in_category( 'nederlands' ) ) { ?></span><?php }
                     if ( in_category( 'deutsch' ) ) { ?></span><?php }
                    }
					 	
?>

<?php endwhile; ?>