To change the language of most aspects of the calendar on the front-end, try using a two-character locale code: [piecal locale=”pt”] should work to translate the calendar’s UI (with a few exceptions) into Portuguese.
The next version of Pie Calendar will have more robust support for string translation, allowing you to translate the rest of the plugin’s strings (including those that don’t change when the locale code changes) using something like Loco Translate.