A problem that I recently encountered was the inability to modify
Dataverse tables. Whenever I attempted to save any changes, the save operation would fail with the error "The table could not be updated. LanguageId should not be null". The screenshot below illustrates the error when trying to add a new column.
Other operations would result in variations of the same error. For example, adding items to a choice list gave the error "The choice could not be updated. LanguageId should not be null".
I could find no obvious solutions online, so I raised a support ticket with Microsoft through the following link.
Following the initial contact with the support engineer, my ticket was reassigned to a team that specialized in Dataverse.
I soon heard back from another engineer who told me that this was a known issue. In my case, unfortunately, the exact cause was unknown. There was no way for a user to fix this type of problem and Microsoft would have to prepare a fix for me. They raised an internal "IcM" ticket and about 24 hours later, I received an email to say that the problem had been fixed.
The outcome of this is that if you receive the error "language id should not be null" in Dataverse, the most expedient course of action would be to raise a support issue as soon as possible.