Processor doesn't work with custom langcode

You can use the language event that gets called [https://git.drupalcode.org/project/snowball_stemmer/-/blob/2.x/src/Stemm... to do just that.

The module uses it, so these are examples of implementations of the event https://git.drupalcode.org/project/snowball_stemmer/-/tree/2.x/src/Event...
One of those subscribers does in fact do what you want (almost) https://git.drupalcode.org/project/snowball_stemmer/-/blob/2.x/src/Event...

Because the (I think IETF) way of writing language codes is 'en-gb' (British English), 'en-za' (South African English), 'de-at' (Austrian German), 'fr-be' (Belgian French). Drupal an example 'pt-br' in it's standard language list https://git.drupalcode.org/project/drupal/-/blob/10.1.x/core/lib/Drupal/...

With the event your, I think site specific as it's the other way round case, can be covered by implementing the event subscriber in a site module.

Rather than do that do you want to try this:

diff --git a/src/Stemmer.php b/src/Stemmer.php
index fd9129e..56e6bf0 100644
--- a/src/Stemmer.php
+++ b/src/Stemmer.php
@@ -70,7 +70,7 @@ class Stemmer {
   public function setLanguage($language) {
     try {
       $language_event = new SetLanguageEvent($language);
-      $this->eventDispatcher->dispatch($language_event);
+      $this->eventDispatcher->dispatch($language_event, SetLanguageEvent::LANGUAGE_CODE);
       $this->stemmers[$language] = StemmerFactory::create($language_event->getLanguageCode());
       $this->language = $language;
       return TRUE

(also attached as a patch file).

Many thanks for reviewing that. Sorry for the delay, now pushed into the 2.x branch. I'll make a new minor bugfix release.