how do i change the UI logic pro x to spanish?

To set Logic Pro X in Spanish, open System Preferences > Language and Text > Language, and put Spanish in the first place. Then, open Logic Pro or restart.

Most of the applications use the OS X language by default. Logic Pro X does this, so you just have to change the OS X language. If you want to change only the Logic Pro language, you can use an application like Language Switcher

In system preferences you need to change your primary language to Spanish, the computer will ask you to restart. This worked for me ( I changed from Spanish to English, but you can just drag the language you want as primary language up.)

Note:

There is currently a bug in the recent Logic Pro X 10.2.1 update, when using Spanish as the main Language...

Using any of the Midi Transform functionality, will crash LPX unfortunately..

Hopefully this bug will be fixed in LPX's next update...