After merging of the patch: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/564789 a few comments came from @Krinkle
ILanguageConverter.php

1. needs @ingroup.
2. Also, being a new Language-related class ILanguageConverter should be in includes/language instead of the old languages/.

LanguageConverterFactory:

1. needs @ingroup.
2. need license header

TrivialLanguageConverter.php

1. need @since

LinkHolderArray.php

1. should be marked @internal and announced in the release notes.

This is a breaking change. Assuming that is intentional, please mark it as such in the release notes and notify Wikitech-l.
Alternatively, remove it from the public interface by making as @internal to parser/ (and add release notes).

It's technically a breaking change per the old policy, but it's unused, and wouldn't be part of the public interface per the new policy. I don't think announcement on wikitech-l is necessary.