Someone on Shanghai Expat recently asked about how to change the language settings on Picasa. Their Picasa was in Chinese, but the user doesn't read Chinese, and didn't know how to go about changing the language. The Solution:

Go to Tools (2nd to last menu Item at the top)
Options (Last Choice of the tools menu at the bottom). Click on that.
You'll see 5 checkboxes & two dropdown menus.

Use the 2nd dropdown menu to change the interface language to your preferred language.