I have tested here and it does not reset the language for an existing article.
The result of the patch is ONLY to present in the language dropdown the current language as already selected when creating a new article, a bit similar to what we get when we install a custom distribution and localise.xml <forceLang> is set, except evidently that in this case here, it does not change the language UI.
In case of validation error when editing an existing article, I guess the article is just not saved.
Please test.

I nevertheless added the check you proposed above to make sure it only applies to a new article.

In fact, the only validation issue I found is letting the new article title empty.
Therefore, I first changed the default language as proposed by this patch to another language, then let the title empty and clicked on Save. Got the error message and then checked the language: it was still set to the language I had chosen in the dropdown, it was NOT reset to the default one.

Hi JM

Looking at your original code again, I see that the code you proposed just set "default" value for language field, not set the value of the field, so my comments were invalid. You can remove that check, it is not needed.

I still think your check is a good improvement.

One more tester :)

I tested it in my website. Now it is working as expected. Thank you very much for your valuable time

Please RTC

RTC based on testing by @anusiram and @joomdonation Thanks!