In addition to thermal insulation (resistance) you should also look into thermal inertia (mass), which is what provides cooling on hot days and moderation on cold winter nights.
Insulating the wall throws away most benefits of inertia, and you have to actively manage the temperature, which usually means spending money to heat and cool.
But you would insulate if the average temperature achieved by inertia alone is unpleasant (usually too low) and costly to manage, as in cold basements.
Whether you choose for inside or outside insulation depends on the specific climate and moisture conditions at your dwelling. Both types are well studied.
OP is advised to get local engineering advice. It's a pricey undertaking and if done wrong it can have dire consequences. Both can be generally viable options, as is relying entirely on inertia.
Here is an example of internal & external insulation practice common in a moist and rainy climate region in North America:
For a question like this, and whether it applies to retrofitting a massive concrete wall somewhere in Portugal, I would not just go by a single article or a web calculator.
But this does provide some background information that should help you ask informed questions.
Images from www.bchousing.org publications IG-R22-Effective-Walls-Residential-Construction.pdf