i went to Andorra in January. Andorra has no reciprocal arrangements with UK mobile providers, so I had no GSM service at all for the week. however so many outlets now offer free wi-fi, eg bars, cafes, municipal premises, etc, that i was rarely out of reach of somewhere that I could use whatsapp.
it's likely you'll find the same, and in the main (unless you need to make a call), you shouldn't need to use your data.