There’s no such thing as a useless skill (that’s not an invitation to come up with some ridiculous examples!)
Don’t overlook the fact that being able to communicate fluently in another language is a hugely useful life skill. They say the first foreign language you learn is the most difficult. After that your mind is conditioned to picking up another. It will also subconsciously make you more aware of the nuts and bolts of your own language. It is widely accepted that language learning helps develop strong cognitive skills, such as a better concept formation and mental flexibility. Now that is very relevant.
As for you specifically Portuguese, I don't know but don’t rule anything out. The Army is engaged in operational duties across the globe. The work ranges from peacekeeping to supporting allies in their anti-terrorism measures and helping combat the international drugs trade. It’s not out of the realms of possibility you might one day be tasked to train or work alongside Brazilian counter narcotic forces for example. Also it may enhance your ability to engage and improve interoperability with our allies on various exercises.
There are options available to those with language qualifications/ skills to train and become an in-service interpeter, the details of which are explained in the relevent MODNET that you’ll be able to access once serving. There seems to be two key languages that are always in demand French and Arabic but other priorities include Russian, Chinese, Pashto, Dari, Spanish and Somali.
The ability to speak to the local population in their own language is an invaluable asset and knowledge of the local language and customs can make a significant difference to the success of both operations and overseas engagement, including on military training teams.
If you have the ability the speak another language make in known to your career advisor and Unit Chain of Command via your JPA (intranet-based personnel administration system) once serving. Once serving search on the intranet for The Army Language Strategy.