— You know, you only speak in Korean when you talk in your sleep.
— I do?
— Yeah. You never sleep talk in English. You only dream in Korean.
— I didn’t know that. You never told me that.
— Most of the times I think it’s cute. Sometimes, I don’t know… I guess I get scared.
— Scared of what?
— You dream in a language that I can’t understand. It’s like there’s this whole place inside of you where I can’t go.

Before anything, here I will not talk about the love language of your partner — but rather their native language. So, beware of it.
Language acquisition is a serious mastery. It requires dedication, hard work and determination because it needs effort and time. After certain age range, it becomes even harder because “a grammar-learning ability that is preserved throughout childhood […] declines rapidly in late adolescence.”
There are numerous examples of people who pick up a language later in life, and our ability to learn new vocabulary appears to remain constant, but most of us will not be able to master grammar like a native speaker — or probably sound like one either.
— At What Age Does Our Ability to Learn a New Language Like a Native Speaker Disappear? by Scientific American
And some language are harder to learn because they are so different than our native language. The language columnist for The Economist points out a list that the US State Department, which teaches new languages to their English-speaking diplomats, uses to divide languages into four categories based on the initial time period they require: 24–30 weeks, 36 weeks, 44 weeks and 88 weeks. As you may guess, almost all of major European languages such as French, Dutch and Italian fall into the first category (the only exception, for sure, is German which fits into the second category). The hardest ones, unsurprisingly, are the most different ones: Chinese (Mandarin), Japanese, or Arabic.
So, all of this begs the certain question: Why are people so enthusiastic to learn their partner’s native language?
Luckily I have always been in international circles where I had an opportunity to witness a lot of cross-cultural relationships or even marriages. And because of that, I know a Lithuanian lady speaking fluent Turkish, an approximately-45-years-old man speaking intermediate Polish (don’t criticize him, he got married recently!), or a lot of young people who started their language acquisition because of their partner in less than a year ago. And I always wondered the reasons behind phenomena (maybe because I never had to learn the language for that purpose — thankfully).
However, when I was doing my research about the phenomena — I came to a surprising conclusion. People learn the language of their partner for the same reason I did certain things for my same-language partner: interest in their past.
Love, after all, is only a superior kind of curiosity, an appetite for the unknown that makes you bare your breast and plunge headlong into the storm.
— Gustave Flaubert
When I was visiting the education centre my partner used to work in her hometown, I stood there for a while and looked around a little bit. Simply because I wanted to feel or even internalise the environment better — the place she stood inside for hours, paths she walked to arrive or depart and so on. I wanted to visualise moments or experience of her from the times I didn’t know her yet.
Even though it took me while to visit there and she got a little bit sad that I never take her off the work (at the time she wanted to have that experience at least once, since we were living in close but different cities back then), after the visit I immediately told her that that reason alone —of course, after her wish— would be more than enough for me to visit there. By the way, if I may say so, I recently visited her new workplace quite often.
Not only past, but language acquisition could also be a way of showing your intellectual investment in your shared future as well. And I mean, after all, whose parents wouldn’t like a partner who showed demonstrated interest in their culture and learned a language to have a chat with them in dinner?
While it’s one way of showing respect to your partner, it’s by no means disrespectful not to learn your partner’s language because language acquisition isn’t everyone’s cup of tea as I cited certain reasons above.
So, long story short: I think, if we have true love for our partner, we’re genuinely interested in their past as well — sometimes, such curiosity could translate itself as an attempt to learn your partner’s native language.