how to preserve interface language for a multilingual website?
Your best bet might be the Administration Language module. It allows you to set the general and user specific administration language. The module doesn't seem to be maintained currently/anymore though and there's only a dev version for the Drupal 7.x branch so you may want to check the bug reports and test it first. I've used it on a few sites without running into any major issues so it should be worth a shot still.