There are quite a few accounts of language attrition to be found in popular fiction. For example, in Volume 5 of Alexander McCall Smith’s Botswana Ladies’ Detective Agency series, The Full Cupboard of Life (2003), the main character, Mma Ramotswe (the Lady Detective herself) reflects on someone she once knew, who left Botswana for Mozambique where he spoke Portuguese and Tsonga, and when he returned to Botswana, to Mma Ramotswe’s shock and horror, he ‘seemed like a foreigner’. She feels that this loss is as sad as ‘forgetting the face of your mother’, and akin to ‘losing part of one’s soul’.
Similar feelings of shock and unpreparedness are expressed in Peter Høeg’s novel Smilla’s Feeling for Snow (1992). Smilla, the main character, a geologist and expert on ice, tells of an expedition to the Arctic where they were field-testing stainless steel snaplinks. After those had been exposed to the hostile climate of the Arctic sea for a couple of months, the steel had corroded to the extent that she could pick them apart with her fingernails. This, she said, is exactly what had happened to her native Greenlandic language.
If you know of any more examples, please let me know.