Here’s how to change the keyboard language on your Apple Watch.

Enable the language on your iPhone keyboard (Optional)

Skip the step if you already have the language on your iPhone.

First things first, enable the language you want to use on your iPhone keyboard by opening your Settings app and following these steps:

• Tap General > Keyboard.

• Tap Keyboards and then pick Add New Keyboard.

• Select the language in the Other iPhone Keyboards section. The new keyboard language will pop into your Keyboards list.

If you want to add more than one new dialect, just follow these same steps to add additional languages.

Change the keyboard language on Apple Watch

Strap on your Apple Watch and open the TGWatch app. Then, follow these simple steps to change the language for a message:

• Open the TGWatch where you want to reply message, which will pop up keyboard input or Scribble/Dictation to input.

• Swipe up from the screen bottom, which show SPACE with an up arrow.

• On the next screen, select the language you want to use.

Reference link: https://www.idownloadblog.com/2018/10/01/change-message-language-on-apple-watch/