Were you able to work around it? I'm trying to translate without success.

No, we are still having this issue.
Here are the steps to reproduce:

1. Setup a user in okta and remove their assignment from application.
2. Login using the user credentials to confirm you see the error message "User is not assigned to the client application."
3. In the web app, set the property for i18n.en.idx.error.user.not.assigned.to.app to "My custom error message"
   i18n: { en: { 'idx.error.user.not.assigned.to.app': 'My custom error message' } }
4. Repeat step 2. We are still seeing the message "User is not assigned to the client application." instead of the expected message "My custom error message"

Thanks for submitting this issue.
I debugged current response for this error and noticed the key name is idx.error.code.user_not_assigned.
Please try it instead

I gave it a try and idx.error.code.user_not_assigned doesn't work as well. Here is an interesting observation: the default values for idx.error.code.user_not_assigned is You are not allowed to access this app. To request access, contact an admin. and the default value for idx.error.user.not.assigned.to.app is User is not assigned to this application. in https://github.com/okta/okta-signin-widget/blob/master/packages/%40okta/i18n/src/properties/login.properties

However, the error message that we are seeing and trying to override is User is not assigned to the client application. which is different than both the custom label defaults.
Also I noticed a unit test in https://github.com/okta/okta-signin-widget/blob/master/test/e2e/features/oidc.feature line 10 has the same message that we are seeing.

It means you are using classic engine. Error access_denied is not localisable for classic engine.
Internal ref: OKTA-595567

Got it. Thank you.

@margish
Can you upgrade to OIE engine?
Or do you have a requirement to have localized access_denied error in classic engine so we can fix it?

We are planning on upgrading to OIE.