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; ?>

Why

Webmentions were shown as “Peter mentioned this on ruk.ca”, but that does not provide context for a reader to decide if they want to click to go there. So I want to display a part of the linked item, as a teaser. Years ago pingbacks were presented like that on my site.

What

Find out how webmentions are stored, and how they are presented. Change the presentation by displaying an excerpt.

How

It turned out that webmentions are actually stored in full (so the text of the linked item is available in my wordpress database). It was just that the template for how it got presented needed tweaking. There is also a character count limit, below which a mention is indeed shown as I like it. All this however isn’t done in the theme, but in a plugin, called Semantic Linkbacks and in a specific file: class-linkbacks-handler.php. Once I localised that, I could change what I needed.
So I

1. changed the character count limit to 500
   // Mentions with content less than this length will be rendered inline.
define( 'MAX_INLINE_MENTION_LENGTH', 500 );

2. changed the template for a webmention in public static function get_comment_type_excerpts()
   It was 'mention'=> __( '%1$s mentioned %2$s on href="%3$s">%4$s., 'semantic-linkbacks' )
   I changed it to
   'mention'=> __( '%1$s mentioned %2$s on href="%3$s">%4$s: %5$s', 'semantic-linkbacks' )
   This adds a 5th parameter at the end. This will hold the excerpt.
3. Then filled that template in, in function public static function comment_text_excerpt
   • After it sets the length of the webmention, stored in $text in $text_len I evaluated that to see if it is longer then the limit. If so it sets a delimiter $text_delim to that maximum, or else sets it to the actual length.
   • Then I create a text snippet from the start of the text to that delimiter $text_snip = substr($text, 0, $text_delim) and add a few dots to the end $text_snip .=" ...."
   • That snippet is then added as 5th parameter to a call of the template mentioned above $text = sprintf( $comment_type_excerpts[ $semantic_linkbacks_type ], get_comment_author_link( $comment->comment_ID ), $post_type, $url, $host, $text_snip);


ToDo