You didn't give us much information to go on which makes it all but impossible to give you good advice. Most notably, what carrier and country are you going to use the phone in.
If you meant TMB US, Portuguese isn't a language in common use in the US and as you've discerned isn't available. Nor do I think that editing languages.xml will work as I believe that is depreciated in the current Android version. The easiest way to do what you want would be to root and install a 900F based custom ROM that has Portuguese support enabled. ROMs such as Alliance or Phoenix probably fall into that category.
In the alternative, you might be able to get Portuguese support by flashing a full 900F stock firmware to your phone. However flashing firmware from a different variant is not recommended and could soft brick your phone. If you proceed anyway, make an effort to find the matching EUR_Open PIT file and flash both concurrently.
All in all, a custom ROM is the better way to proceed.