How to exclude Admin Toolbar from being Translated in Drupal 8?
You can set a separate language for administrator pages on the account edit page for each administrator account.

Under Language settings, set the Administration pages language.

BUT! There is a major core bug related to this feature: if admins are creating fields, the config will be screwed up.  See the issue for workarounds.

Admin toolbar module

If the question is specifically about the Admin Toolbar contrib module, you can not download the translation for it.  If no translations are downloaded, the module will always show in English.