Android
To change the language:
- Open Signal.
- Tap your profile to go to Signal Settings.
- Select
Appearance. - Select the last item Language.
- Scroll through and select the language for it to apply immediately.
iOS
iOS 13 & iOS 14
To change the language:
- Follow steps from Apple's support page to ensure the language is enabled in iOS Settings.
- Go to
iOS Settings > Signal. - Select Language.
- Scroll through and select the language for it to apply immediately.
iOS 12
Signal matches the language on your device. Follow steps from Apple's support page to change the language.
Desktop
Signal Desktop matches the language of your computer's operating system.
To manage spell check:
- In Signal, tap your profile
> 
Preferences... - Under Chats, select or deselect Enable spell check of text entered in message composition box.