I had a similar issue - I wanted the site to be in German but the admin (interface) language to be configurable between German and English. However, when adding German he didn't import the translations, it said 0/10000+ of the user interface was translated so everything stayed in English. Running the language update didn't help, neither from the command line with drush nor through the admin interface.
I suspected that missing permissions on web/sites/default/files/translations could be the issue but it seems to be something else.
Looks like a weird issue probably connected to ddev but I couldn't really get to the root of the problem.
However I found two workarounds:
- Delete the language via the admin interface and add it through drush with ddev drush language-add de (or en): Then he did actually import all the translations
- Manually download the translation file (e.g. drupal-10.2.6.de.po) from https://ftp-origin.drupal.org/files/translations/all/drupal/ and import it via ddev drush locale-import de /var/www/html/drupal-10.2.6.de.po - however that only covers the translations of drupal core, for other modules you'd need to download their translations manually as well and import them
Maybe someone else will have more insights...
Also for the interface language to be configurable in each user's profile make sure you have that enabled in Region and language > Languages > Detection and Selection: Account administration pages
holybiber → created an issue.