Setting session.props.locale in the Designer does not set the locale of running sessions on start because that setting is set at the project level:

The session.props.locale property allows for changing the locale at runtime. So you have three options:
- Change the project-wide setting to force all sessions to open as a certain locale.
- Change your browser to the locale you want to use.
- Allow for sessions to set their own locale by supplying some sort of input (Dropdown is common) which is bidirectionally bound to the
session.props.localeproperty.