The "Basic translation for default Language" switch, originally located on the Manage Translation page, has now been replaced with adding a translation language as the default language in the language list.
If you want to disable translation for default Language, simply delete the default language. Please note that this will not remove your original language content.

This feature is applicable when: the default language selected for your theme does not match the language filled out in your product information, and you wish to display the default language.
For example: If your store's theme language is set to English, but your product information is written in Chinese, and you want to present your store in English to users, you can activate this feature.
Please note: When a translation language is set as the default, editing the translation results is temporarily not supported. You will need to edit the original language content on Shopify.
For users who haven't enabled the feature, you can activate it by navigating to the Language page, clicking on 'Add Language', selecting the default language.