"Language with ID '0' does not exist." - error after migration

[maciejdoboszadhd]

Brief bug description

After running the Kentico Migration Tool with no custom modifications, some migrated pages throw an exception when opening the Content tab in the Xperience admin.
The error shown: "Language with ID '0' does not exist."
This results in an UnhandledException (System.InvalidOperationException) coming from ContentLanguageRetriever.GetContentLanguageOrThrow.

Output logs

protocol20251124_0827.txt

Repro steps

1. Open the Xperience admin.
2. Navigate to Pages and select a migrated page.
3. Click the Content tab.
4. The error dialog appears instead of loading the content editor.

I have no idea why some pages are working and some are not :(

Expected behavior

The Content tab should load correctly for all migrated pages without throwing an exception.
All languages should resolve properly, and no references to language ID 0 should occur.

Test environment

Platform/OS: Windows 11
Browser (e.g., Chrome, Edge): Chrome
Kentico Xperience version: 30.10.1
Migration Tool version:3.16.0

Additional context

No custom migration code or custom modules were used; issue occurs on a clean migration.
It appears some content items or references may be missing language assignment or default language resolution after migration.

Screenshots

[akfakmot]

@maciejdoboszadhd Hello, this is a data specific issue. Would you be able to provide either full database backup or at least a some restricted version of it where you'd delete unrelated data that you don't want to share, but still keep data related to the issue?
On top of your own data prunning, you can use our standard anonymization script:

KX13_simple_sanitization.sql

[maciejdoboszadhd]

Okay, I will try to give you some data

[akfakmot]

Hello @maciejdoboszadhd ,

we've been able to simulate some problems with language handling. The problems were fixed in the latest release.

https://github.com/Kentico/xperience-by-kentico-kentico-migration-tool/releases/tag/v4.0.0

It's not 100% clear that this was the source of your problems, but there's a good chance.