Did you know that Spark Mail on macOS sets its language automatically based on your system language? However, it may not support all language variants. For instance, if you're using Traditional Chinese as your system language, Spark Mail will default to Simplified Chinese. Fortunately, there's a way to change the default language setting! Follow these steps: Open Terminal, identify your Spark Mail installation type (App Store, website, or Setapp), and run a specific command to update the language setting. Finally, restart Spark Mail to apply the changes.

Source: https://dev.to/lichieh_su_ce93b64a0eee69/how-to-change-the-default-language-in-spark-mail-on-macos-225e