I tested the plugin and it works fine.
You are able to translate single cells or the whole sheets tab.

When you use the formula, the advice is to copy the translated text with ctrl+c and insert it with crtl+shift+v because the translations will otherwise update every time you open the sheet.

The regular pricing is 4,- for 1000 words which equals a A4 page.
In this offer you get unlimited translations over DeepL Api.

I dont know how this offer could be sustainable and hope the service survives!