I spent three years out of my country, and I observed other Americans who worked with me. It appeared to me that everything depended on whether or not you assimilated linguistically and culturally to the host country. The people I worked with never really learned the language of our host country, because they were very cagy about finding other expats to talk to, and finding locals who spoke English well. They read and listened only to English media, and spoke only English. I don’t think these people saw any change in their vocabulary.
When I got to the place, before most of them, there was almost nothing to read in English, and almost the only electronic media we had in English was CNN International, which, besides being a 24-hour propaganda bath, had a lot of non-native English speakers as reporters and announcers. (CNN in the US doesn’t have these people, and other cable news stations are much more popular than CNN anyway.) There was some MTV, but it was MTV Europe, which meant that it had a combination of Dutch, Swedish, Belgian and German announcers who spoke English well, along with some from England who spoke horrible English. I also did not use English as a criterion for choosing my friends, so most of my friends there spoke little or no English.
Anyway, there was little English input for me.
I did not lose my vocabulary, but some of it went dormant. The first things that got put into “storage” were the names of famous people in my home state and country, and the lyrics to songs that I knew by heart. Some puns and funny slang also went dormant, because even the local people who understood English did not understand double meanings or any kind of word play, so some of that stuff also got put back in my mental “warehouse”, and I couldn’t retrieve it when I wanted it. All of this stuff eventually came back, and more, after I got home.
The biggest problem for me was not losing vocabulary, but picking up concepts from the new language that didn’t exist in my own. Czech lacks specific words for huge numbers of everyday American concepts, but it has words for other concepts that we don’t express in English but that I found useful. When I would speak English I would get stuck when I had a concept in my head that English had no word for. I could use a circumlocution, but sometimes people here don’t think about these concept much, and so they don’t really understand them well. English, for example, has no generally used word that describes a personality quirk that’s caused by someone’s job. We have no word for getting pleasure from the misfortunes of other people, so we have to use a German word that is part of English, but that most Americans don’t know. This is cultural, because almost no Americans sit around imagining that their lives will be better if someone else’s life gets worse. It’s a relatively unusual emotion here, but in my host country I encountered it every day.
Another thing that can happen is that your own language can change while you’re away. When I left the United States, the S-word and the F-word were much more obscene than the N-word. By the time I got home, the S-word could be used in magazines, and the N-word had become completely taboo for white people to say (blacks continued to use it among themselves). One time I tried to use the N-word in a university classroom to describe two different classes of slaves before the Civil War. When I was in school, we knew that the expressions I used were bad words, but that they were the standard terms for certain slaves’ jobs in the old South during slavery days, so teachers of all races used them. By the time I got back to the US, the N-word had become so obscene that I could not use it in any classroom even to denounce it and describe an injustice!
Another example of this: Once I had to work in a recording studio as a coach for a Czech man who was doing a voiceover for a video. This man had escaped from Czechoslovakia in 1968, and he hated his homeland so much that he didn’t want to go back even for visits, and even after it became free. The text he had to read in the recording booth contained the Czech word “akcion?r”, which means “shareholder”. He wanted to change the word, because he remembered it as almost an insult that was used under communism to describe capitalists. I had to explain to him that it is now the normal term and was not derogatory or insulting.
All in all, my biggest problems upon returning home were not with language, though. I had “gone native” in my host country, so I had a lot of trouble understanding my own people and culture when I returned home, and 11 years later I sometimes still have problems.