I've had a big reported to me where a user of my watch face with his watch set to Italian only shows D rather than DOM for the day of the week display. This occurs on a Garmin 7X with the language set to Italian. This also occurs on the "stock" pre-installed watch faces as well, so I suspect that this may be an issue with the firmware/OS.
I have tried to re-create this big on my own watch in addition to the simulator and have been unsuccessful in replicating this issue.
I am using Time.FORMAT_MEDIUM for the day format, so I would not expect this issue.
