Drupal 8 is the best and most translatable Drupal ever. The D8MI made an awesome process in getting the biggest pain points done.
However, there are still things that can be improved for Multilingual Drupal 9.
Some of them:

- Revisions and Translations bring the issue that an update of an entity for a single language creates a new revision which is for all languages. This could generate issues with workflow systems which work with revisions.
- Translatability is key for Drupal, but big multilingual websites need translation workflows, like assigning of translations, review of translations, status tracking or giving translations to external partners
- In Drupal 8 there are three different UIs for translating: String, Config Entities and Content Entities. This is complicated and generates overhead for translators.
- A look how other CMS and Frameworks do translation and maybe some things we could use from there.

I will present these issues deeper with real life examples and possible solutions.