The default language in Glotio is the source language whence all the translations begin. This is very important because Glotio understands that your website content started from that language in your PrestaShop. Glotio only recognizes the ISO code you set.
The default language in Prestashop is set up in the "Localization" section:
IMPORTANT: Changing the default language or any other language in your PrestaShop after doing some translation with Glotio, it means that the tool will stop working/translating your texts until you set up again the default language such as at the beginning. Changing or deleting the default language doesn't allow to reactivate the launch promo of the first language.
Take a look to this article: Error: Default language changed, Remote language deleted
You can see your default language and the languages already activated in your PrestaShop in the "My Languages" menu. Here you can see those that are already translated, those that are translatable and those that are being updated.