Ofcourse if a invasion takes place and all the natives are killed or reduced to a negligible population then the native language may dissappear but as long as a sufficient population of natives remain the language will survive and if it has to will die a slow and gradual death.
I believe I am correct about Lombards and English, and 50 years in those days, particularly for a tribe in such circumstances, would be about 1.5 generations. I did not state they had lost all their German words in 50 years, simply that they did not speak a Germanic language. Probably a hybrid that soon, by the historical records, had almost no German words left.
Some more of his nonsense. In his book "Реконструкция всеобщей истории" (Reconstruction of world history), he states that Pope Innocent III (1160 - 1216, living in Rome) and Kniaz (prince) Ivan Kalita (1283 - 1341, living in Moscow and Novgorod) are the same person. I use them as an example because they are verifiable real people with dates of birth and death and what they did in life.