The most common reason is that the theme you use might not be fully compatible with Shopify native translation feature, so it does not pass the language info to the checkout page, that is why the checkout page stays in your shop default language. You can go to the "Language Selector" page in the App and add the selector to your website. The language selector can fix this issue.
Another common reason is that some Apps hijack the checkout button, so it blocks Shopify from sending the language info to the checkout page. Thus, the checkout pages do not know the selected language, and they always use default language even the translation text are available. If this is the case, please contact us at email@example.com, including your store URL. Then we will check that and confirm it for you.