Now, the auto-swap checkbox: it appears if and only if you set the auto option in "Source language" combobox, and is directly connected with "Language for swapping" combobox.
Obviously this function has a major or minor utility according to your more frequent needs. So, imagine this situation: you usually translate into English (your main language), but sometimes (for example, when you write a document) you need to translate into Italian (your second language, suppose) you can set "Language for swapping" combobox to Italian, so you will translate from English to Italian without accessing directly to the addon settings. The "Language for swapping" combobox is useful when you swap via script (see below) the source and target language in fact, a target language set on the auto option has no sense, so the addon sets it to value of combobox above. Remember always that the explanations below assume the source language set on the auto option.
The meaning of two first comboboxes and checkbox for copy is clear, but some words about the rest are necessary. In addition, if you selected auto option (the first choice) from "Source language" combobox, there are also a combobox labeled "Language for swapping" and a checkbox about the auto-swap. There are two comboboxes labeled "Source language" and "Target language", and a checkbox to decide if it must copy the translation to clipboard. To configure source, target and in case swap language, go to: NVDA Menu > Preferences > Instant Translate Settings. This is done using the Google Translate service. This add-on is used to translate selected and/or clipboard text from one language to another. Authors: Alexy Sadovoy, Beqa Gozalishvili, Mesar Hameed, Alberto Buffolino and other NVDA contributors.