We get about 2/day of this error (example). Some user agents:
Mozilla/5.0 (iPhone; CPU iPhone OS 13_6 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1.2 Mobile/15E148 Safari/604.1 Mozilla/5.0 (iPad; CPU OS 12_5_5 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.1.2 Mobile/15E148 Safari/604.1 Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1.2 Safari/605.1.15
Caniuse says Intl.locale is supported in Safari on iOS 14+. Grade A support covers Safari on iOS 9+. So, we should probably do something about this, but not sure what.