You cannot mark your own translations as checked. This is an optional quality assurance feature and it is not a requirement. But feel free to ask other Ukrainian translators for feedback.
Am I prohibited from creating another account in order to check the changes made by me?
Yes, that would be against the intention of the feature.
Why would you want to do that? You are free to do another pass on the translations and correct any mistakes you notice and to ensure consistency, but without marking them explicitly as reviewed.