My wife and I lived almost eight years in Belgium. I learned enough French that I could get in trouble with cab drivers. And trust me, I really tried. But French is a language that just goes in one ear and out the other for me. Unlike Spanish or Italian.
My wife learned enough business French that she was able to survive, but never became fluent.
There are just some people who cannot learn the local language, no matter how hard you may want to force them to do so or how long they've been living in that country. And I think it's a huge mistake to try to force them, or to judge them poorly for being unable to warp their mind around something that is a fundamentally alien concept for them.
As a linguist, the evidence is simply that your wife (or whoever) simply did not get enough exposure to language she understood, at the level she was at. Unless someone has a true learning disability, the only way one doesn't learn the language is because they weren't able to get the right input. And I say that without any blame - for whatever reason it was, that is the cause.
My wife did much better than I did, even though she never became fully fluent.
I will agree that I didn't go to a full immersion language school. But we did arrange for multiple tutors, some on our own dime and some paid for by her employer or mine. And we interacted with the people in our neighborhood on a daily basis.
I found that those who were Flemish speakers would actively prefer to practice their English with me, while Francophones would usually switch to English grudgingly, once they discovered how bad my French was. There were a few Francophones who couldn't speak English at all, or who refused to switch, and interactions with them were very limited and not very productive.
I honestly tried to learn French, but none of it stuck.
In contrast, during those same years, my wife and I took three different vacations to Italy. Twice to Rome and once to Ischia and the Amalfi coast. And I swear to $DEITY that I learned and used more Italian in those four total weeks, than all the French I ever spoke over nearly eight years in Belgium. Something about Italian and Spanish is just far easier for me to pick up.
This reminds me of a recent experience of mine. I was raised bilingual Greek and English (English is my primary language). For my entire life I heard both of these languages and use them both the same. What I had never heard, until recently, was someone who learned Greek as an adult. I was so excited to speak to them because it was something completely new to me.
When they began speaking (Greek) it was like my brain shorted out. I was so excited for them but it took active mental effort to translate what they were saying. I began speaking to them in English and I nearly couldn't force myself to speak Greek.
This was not elitism or snobbery, it was like my brain decided the path of least resistance was English and I used it automatically. Very strange experience.
It's always a balance between efficiency vs willingness to support their learning efforts.
This reminded me of an article written by an American living in China recounting his early days there. In it he describes the tricks he developed to practice the local language despite the higher interest of the locals to practice their English with him.
You must have missed the part where I said that all the Flemish speakers I met wanted to practice their English with me?
I never got a chance to try to learn Flemish. Every single one of those speakers that I met, were also master linguists (whether they acknowledged it or not), and spoke at least six languages fluently, and those were the only ones they admitted to. They also spoke at least two or three other languages with less fluency, and yet they claimed only the ones in which they were fluent.
During the time I worked at Snow, BV (a Dutch consulting company), part of my employment agreement with them was that they would pay for and arrange to have a Dutch tutor for me, in Brussels. By someone who really spoke Dutch, not Flemish. Unfortunately, that never happened.
To my ears, Flemish and Dutch seemed to be the same language. But not to the Flemish people in Belgium -- to them, these are two distinct languages. The Dutch would agree with me.
And to my ears, both of these languages sound like German but spoken with English word sounds. Every time tried to listen closely, it would nearly break my brain.
My wife learned enough business French that she was able to survive, but never became fluent.
There are just some people who cannot learn the local language, no matter how hard you may want to force them to do so or how long they've been living in that country. And I think it's a huge mistake to try to force them, or to judge them poorly for being unable to warp their mind around something that is a fundamentally alien concept for them.