When I lived in Japan, I became halfway fluent in Japanese. After I left, I never made a sustained effort to keep in touch with the language, and now it makes me feel in equal parts sad, frustrated and stupid when I have to slowly parse a simple sentence to understand its meaning.
Sometime back Tommy wrote me an email which involved no maths, but where he used the phrase "Without prejudice or loss of generality". It took me straight back to combinatorics proofs (which I loved), and the wave of nostalgia hit me with surprising intensity, almost like a physical wave. More recently I had the same feeling at a European airport where all the announcements were in English, but suddenly and unexpectedly there was one in Japanese, asking Kanada-san to report to Gate No. ---. (I am not hiding the gate number; by the time I had translated the first part of the announcement in my head, I had missed it.)
Snatches of languages which I'm slowly forgetting seem to trigger a linguistic equivalent of the Proust effect.
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The Brazilian footballer Philippe Coutinho scores a lot of goals with a trademark right-footed curling shot from just outside the penalty area. Last year he moved from Liverpool to Barcelona, and this week he scored just such a goal for his new team. A Liverpool supporter on Reddit wrote an unusually poignant comment: "it's like suddenly remembering that funny thing your ex used to do".
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There are plenty of Japanese words which are said to have no equivalent in English (I have been guilty of invoking some of them myself). It amuses me therefore that the Japanese word for nostalgia, the title of this post, is simply a phonetic rendering of the English word: nosutarujia.