As of the meantime, the option to change the language for QBO US version isn't available. For now, the language settings (e.g Spanish) is only available in the international/global versions of QuickBooks Online.
If you're using the international/global version, Here's how to change it:
However, If you're using the US version, you can change the language preferences in your browser and choose Spanish. The process differs depending on the browser you’re using.
I can see how important and beneficial it's to you and our other valued customers. I'll make sure to gather the information here and tap this to our developer's team. This way, adding Spanish to the US version might be given a chance to be included in the next product updates.
In the meantime, you may want to consider visiting our QuickBooks Blog. From there, you can see the future developments and the latest news about QuickBooks and what our Product Care Team is working on.
Let me know if there's anything else that I can help you with. You got me covered! Stay safe!