Allow default language configuration for apps or show language selector on instantiation-errors

### Description

When a self-identified (level-0) user attempts to instantiate an Altinn app and instantiation fails (e.g., due to policy or validation), the language selector is not available because it only appears after instantiation. In these failure scenarios, the default language falls back to Norwegian (nb), ignoring the user’s language preference in their Altinn profile.
This results in error messages in Norwegian, which self-identified/international users often don’t understand.

Current Workaround:
We currently override text resources in resource.nb.json to English for relevant keys (e.g., instantiate.authorization_error_instantiate_validation_title).

Requested Solutions:
1. Configurable default language per app
2. Expose a language selector on instantiation errors.

### Additional Information

_No response_

No files or tests are named. Start by tracing the instantiation-error flow and where language selection is initialized; clarify whether the intended outcome is an app-level default language, a selector on errors, or both. Done means self-identified users see understandable error messages based on their configured language preference.

Written by the indexing model from the issue text.