Look into your /app/locale/en_US folder. There you have .csv’s that do the translating. Everywhere you see $this->__('Checkout') or any other string, Magento hits up this folder for specific files that dictate what to replace with “Checkout” and every other string which echo’s this way.
In the CSV files, the first field is the text to look for within $this->__(''), the second is what to replace it with. You’ll also find different language translation files here.
Each CSV is named after the namespace it belongs to. Hence, Mage_Catalog.csv contains the translations for all the files that use the /app/code/core/Mage/Catalog/ classes and /app/design/frontend/accessshop/default/catalog template files.
Here, you’ll also find Mage_Adminhtml.csv, which handles most of the admin translations.
For you to translate to portugese:
Check if there is folder named pt_BR inside app/etc/locale if not create and try copying files from the en_US folder and translate via csv.
Also Set the default locale from Backend > System > COnfiguration > General > Locale Options
Most of the contents are from static blocks and pages you can change those from there itself.
If you have any problem then please feel free to write back.