Is this a widespread belief in Portugal? Second time I've seen it on the thread. The generations who grew up under Franco (over 40 years old) are terrible with languages, because Franco didn't want Spanish people speaking other languages, so his education system didn't teach them. However, things have changed drastically over the past 20 years. I taught in a Spanish Masters program for Conference Interpretation for 7 years, and over the years worked with nearly 200 mostly Spanish multilingual students (also German, Dutch, Swiss, English, Irish, Croatian, Romanian, Serbian, Polish and French students), so I have seen firsthand that the younger Spanish generations now are on par with other European countries in terms of learning foreign languages. There are plenty of Spanish true bilinguals (especially in places like Catalonia, Galicia and the Basque country). Developing perfect accents in second languages generally depends on the life experiences of the speaker. People who live for a time in foreign countries at a young age, or have parents who speak to them exclusively in a foreign language from the cradle, often develop perfect or near perfect accents, because "hearing" the sounds of that other language when the brain is still in early development gives them an advantage. The vast majority of people learning a second language in other ways are unable to shed their accents entirely, but it's not like that is limited to Spanish people and the Portuguese are certainly not exempt from this. Just listen to Mourinho (who speaks many languages very well) talk about the Shempiens league.
Also certain mother tongues adapt more easily to certain second languages, so depending on the person (some people just have an innate ability to pick up languages), their mother tongue, the second language they learn, their early childhood experiences, parental influence and education. Your results will vary.