Streak uses a combination of by-hand translation and machine translation. We use Google’s translation engine wherever there are gaps in our knowledge, so that all extension material, labels, and titles are available in your preferred language.
You can configure this alongside Gmail by adjusting the display language in Gmail's Settings, under the General tab.
Use Streak only in English
If you would like to use Streak in English, regardless of your Gmail language settings, there is an option built into the Streak dropdown to the right of the search bar:
At the bottom right, just hover over English and click to confirm the change.