Will an Asus Transformer Prime bought in the U.S. work in Portugal?
The tablets in Asus Transformer line (including TF 101, Prime TF 201, and Pad 300) do not have a cellular modem, so they will only work on WiFi.  WiFi is a universal standard and will work just fine in any country, provided that your router supports the 802.11 b/g/n specification.