Back in Stock supports multiple languages.
If your Shopify account has additional languages included in the Store languages section; This is under Settings -> Languages in your Shopify account dashboard.
Once an additional language has been selected and published - you can visit the 'Templates' page, under Customize(1) -> Templates(2) in the Back in Stock dashboard.
Then in the top right above the preview will be a drop-down that will show any published languages on the store;
This will show a new version of the page, where you can input text and settings for the alternate language version.