One of the most frustrating and devastating aspects of any illness or injury that affects the brain is when our language function becomes impaired. This can be the result of conditions such as a stroke or dementia or of an accident, and the language problems people have can be extremely varied.
For people who speak more than one language, which of them will be more affected by any cognitive impairment, and which will recover more quickly and fully, is impossible to predict. Quite broadly speaking, it is most likely that the language which was the strongest to begin with will be the least affected and the first to recover, but all other possible combinations have been found. However, it has been shown that, in general, multilinguals recover their brain functions more quickly and fully after a stroke than monolinguals.
We do not know as yet whether a language that has more strongly attrited before the onset of any cognitive symptoms will be more vulnerable to aphasia. However, if you notice your language (or the language of someone close to you) suddenly deteriorating, do not put it down to the myth of language reversion, but treat it as a possible early warning sign of something more insiduous.