If your store supports multiple languages, you can translate the text displayed by the app into each language. Where you translate a piece of text depends on its type, and falls into two places.
Configure this from a selling period rule's "Localize" screen. This covers:
Product messages (before, during, and after the selling period)
Alert messages (before and after the selling period)
The countdown timer's title and description
In languages without a translation configured, the default text entered in the rule is displayed instead. You can select any language added in Shopify (excluding the default/primary language) as a translation target.
The countdown timer's "units" (days, hours, minutes, seconds) are not part of the rule translation. Translate them from Shopify's Translate & Adapt app, under Theme > Templates > Product > "Countdown Timer."