Apple Watch SE 2nd gen
watchOS 9
1. Find "Add Language..."
On your phone: Press Watch.
On your phone: Press My Watch.
On your phone: Press General.
On your phone: Press Language & Region.
On your phone: Press Custom.
On your phone: Press Add Language....
2. Choose language
On your phone: Press the required language.
On your phone: Press Use ‹language›.