On Generate Emails, the Generation language picker sets the language emails are written in; you can choose from 46 languages. The choice is saved for that strategy.
If you change the language while drafts exist, you are asked to confirm, because the new language applies to later generations.
To have one email written in another language, open it with Open in Focus Mode and use the regenerate-in-language option.
If no language is chosen, the AI guesses a suitable one from the recipient company's information and the notes in the strategy. If your target market is clear, choosing the language explicitly is recommended.
The app's interface language is a separate setting, changed with Language in the account menu.