Enable Glossary Translation

+    $terms = \Drupal::entityTypeManager()
+      ->getStorage('taxonomy_term')
+      ->loadTree($this->settings['onomasticon_vocabulary'], 0 , NULL, true);

I decided to not load the full entities on purpose. Imagine having a quite large glossary vocabulary, this will result in high memory consumption. So, I tried to fetch a full term entity only if the term name is found in text. In fact, on my test page, I ran out of memory despite having increased the limit to 1024M. And there is only a simple set of terms.
I am aware that ->has/getTranslation() won't work with the partial term objects, though.

+      if (!$term->hasTranslation($language)) {
+        continue;
+      }

This would potentially disable all terms that have no language set.

$this->termCache gets values because in the latest dev version the user can choose whether to only replace the first occurrence of a term (default, indeed no value needed) or to replace all occurrences (in this case the value is taken from termCache and not loaded again from database.

Regarding the translation, I was looking into that as well and neither found any way to just get the available translations without fetching the complete Term objects. We'll need some smoother way to handle this. I just started Drupal 8 a few weeks ago after almost eight years of procedural D6 and D7 and still need to dig through the possibilities of the new system. Also, last time I used Symfony in 2009, it was in major version 1, so I have a pretty steep learning curve right now.

That said, thanks for your kind words and also thank you for the patches provided and all the input. I really appreciate it and it helps me to find my way through the D8 jungle.

Just had a quick look at the new patch. Since you created the new $terms class variable, my remark about termCache is of course invalid and you are right, now it doesn't need the values.

I am unlikely to test it all until Monday due to some deadlines I have to meet, but I will try to do it as soon as possibly.

Couldn't sleep well, so I actually did test it. Very good job, it seems to work quite well for Latin-based alphabets. I was tinkering around the last hours trying to get it work for Russian, but failed so far. But in general, the pure translation stuff works.