From a personal point of view I know that growing up speaking German and Italian while having an English teacher as mother really helps.
But now I'm teaching English myself both for adults and kids, basically everyone interested and the problem I see is that
learning a language is not that difficult.
The problems arise from not using it. Having the endurance and stamina to keep the language up to date and "in working condition" is the hard part. Especially if you don't use the language for work, friends or family.
That's why hobbies (e.g. playing video games) work great.
I was forced to learn French. My parents and I never wanted me to learn French. That peculiar school back then forced me to learn it. Additionally I had the worst teacher in my life, of course it all happened at a very influencial age during my adolesence and - as you probably guessed already - it went horribly, hideously wrong.
Instead of making French palpable I learnt to despise it, to hate it with passion, even to a degree of self-aware and deliberate xenophiba at times.
I've nothing personal against any French at all, it's just a country I don't like anymore. I would never spent my holidays there, even if paid I wouldn't go there. I'm far from incorruptible but trust me you don't have that much money, trust. If there is a video game where the french can be killed first, or sent in as cannon fodder or economically exploited in any way I probably going to enjoy that part.
So back on topic,
learning another language is "rather" easy, but at some point you need support from that language, you need to get involved in that language so that you have the possiblity to exercise it.
Of course there are languages that are more complicated than others, I wouldn't dare to let people learn German (only possible if you don't mind three different articles) and languages with different letters (like Arabic, Chinese, Japanese, Cyrillic etc.) prove an additional challenge. Don't try to learn French by the way, it normally doesn't end well. Did I mention
French military victories??? If you want to sound smart and be nerdy start with some Latin!
Latin is wonderful because most languages, especially English - in fact all Indo-Ayran and of course all Romance languages are derivates and stem from Latin. You even use latin in court, at the doctors or in politics if you want to sound smart.
de pluribus unum, carpe diem, tendovaginitis and pecunia non olet for example.
If you don't want a scientific approach you better start with something utterly useful like Spanish and then switch to Italian because they are vaguagly similar.