I see that it is possible to select language in feed configuration. Does the app do translation to the feed data?

The app allows you to create translated feeds. There are 3 ways to create translated feeds corresponding to 3 types of translator apps that enable multi-languages on the Shopify stores.

• Shopify language API compatible translator apps. This includes Langify, LangShop, MultiLingo, Translate My Store, and other apps not limited to this list. You can read more about Shopify multi-language support here
• Weglot
• GTranslate

This is how to enable your GTranslate:

First, you need to create API token for a custom app.

Second, you add GTranslate Storefront access token you just created. 

Third, you get started create a feed with domain or language you would like to

So these apps provide the translations that Multifeed Google Shopping Feed app use to create translated feed.