While one can certainly only study Russian to get access to a third language or have the goal of using it to read mathematical journals (*more on that, it's very much a real user case...) I'd like to go back to the basics for a second... When I wrote that language is culture and when I see what people are writing about it, it seems that we need to go back to the definition of "culture".
Culture with a small "c" is ...wait let me borrow ... "Culture is a term that refers to a large and diverse set of mostly intangible aspects of social life. According to sociologists, culture consists of the values, beliefs, systems of language, communication, and practices that people share in common and that can be used to define them as a collective." (see https://www.thoughtco.com/culture-definition-4135409
"In brief, sociologists define the non-material aspects of culture as the values and beliefs, language, communication, and practices that are shared in common by a group of people. Expanding on these categories, culture is made up of our knowledge, common sense, assumptions, and expectations. It is also the rules, norms, laws, and morals that govern society; the words we use as well as how we speak and write them (what sociologists call "discourse"); and the symbols we use to express meaning, ideas, and concepts (like traffic signs and emojis, for example). Culture is also what we do and how we behave and perform (for example, theater and dance). It informs and is encapsulated in how we walk, sit, carry our bodies, and interact with others; how we behave depending on the place, time, and "audience;" and how we express identities of race, class, gender, and sexuality, among others. Culture also includes the collective practices we participate in, such as religious ceremonies, the celebration of secular holidays, and attending sporting events."
In that sense, language is culture. It is part of the common semiotic representation. It is part of the system of values that a group uses to communicate. If that group is "Russian Mathematicians" then the culture also included the way language and symbols are used to write a scientific journal (distinctly different than the way a cookbook is written) and it can be studied as a corpus.
Perhaps the other definition of culture, culture with a big "C", the dictionary culture is "the arts and other manifestations of human intellectual achievement regarded collectively."
Clearly, if you are studying to read Turkish language manuals from Russian, one can do so while certainly minimising the access to the arts and other manifestations of human intellectual achievement
. And I certainly understand that point being made.
Excuse me for saying so, but isn't that kind of obvious and sterile? If one is only interested in reading a language, one doesn't need to study how to speak. If one is only interested in translating technical manuals, one doesn't need to study 19th century drama.
But maybe it isn't that obvious. Back to that math example. *This summer, a few weeks ago, I had the pleasure of catching up with a college roommate. His lovely parents joined us for dinner and I discovered that his father, a mathematician had been required to learn German, Russian and French in his graduate program (50 years ago) principally because the cutting edge of mathematics was published in those languages. But the language program did not focus just on reading math publications, it was intended to bring the student fully to what is now called a B2 level. Not necessarily the most efficient use of time, if one is only
going to read technical articles. On the other hand, it's fully in-line with the mission of the school to prepare students for fuller interactions across countries and, gasp
I'll stop here.
(math is culture.)