I found one issue related to Flask-Babel migration.
Flask-Babel and Flask-BabelEx are causing the issue if we install them in the same environment.
Flask-BabelEx gets installed as a dependency of Flask-Security-Too library and currently, Flask-Security-Too doesn’t support Flask-Babel, but the latest version (4.0 Not yet released) of Flask-Security-Too will shift to Flask-Babel.
So I think we should hold Flask-Babel migration for now. but we can update the FLask-BabelEx to the latest version as there is one issue related to it this https://redmine.postgresql.org/issues/5997 we can fix it first by adding the Flask-BabelEx minimum version in our requirements.