Thanks for reaching back out to the Community and adding some extra clarification to your question about if there's an API that allows you to change the language on your sign-in page. I've got you covered.
I can see how beneficial editing the language could be for you. I recommend contacting the Intuit Developers Support Team. They'll be able to answer any questions about API and give you a better solution for your business.
Scroll down and choose the product you're using. (QuickBooks Online)
Select the Ask a Question button.
That's all there is to it. You can also check out our tutorials page to see more details about other features in your QuickBooks Online (QBO) account.
Let me know if you run into any trouble along the way. It's my priority that you're able to get an answer as soon as possible. I'm only a post away if you need me. Wishing you and your business continued success!