Where can I translate the "One-time purchase" text?

You can translate the “One time purchase” text in Seal Subscriptions > Settings > Texts & Translations. In there, the text listed will be the “One time purchase” text. To change it, simply change it's value in there.

And if you want to translate it in multiple different languages, just use the language switcher at the top of the page and set a different text for each language.