Hello, alvarodiez.
Thanks for turning to the Community for support. I'd be glad to share some information about changing the language in your QuickBooks Online account.
This setting can be changed in the Account and Settings section but is only available in the international version of QBO. Here's how:
1. Click the Gear icon in the top right corner.
2. Select Account and Settings.
3. Choose Advanced.
4. Go to Language and click the small Pencil icon.
5. Click the drop-down and choose Spanish.
6. Select Save when you're finished.
That will do it. Please know I'm only a post or comment away if you have any other questions.