Select Tools > Options. Then click the General tab, choose a language from the drop-down menu, and click OK, restart Picasa.
When you cannot read the menus:
1. On the top toolbar, count from left to right until you get to the 7th (last but one) menu item (Tools).
2. In the Tools menu, scroll down to the last item (Options). The Options screen has 7 tabs.
3. On the left side of the Options window, select the first tab: General. The General tab has 7 small boxes and two long drop-down bars.
4. Select the 2nd drop-down bar, then scroll to the language you want and select it.
5. Click OK.
6. Close and restart Picasa to see the new language.
Changing the language setting is in Apple > System preferences.
Note that any change you make will apply to all applications, not just Picasa.