Germanic conquerors may be first partially reverse-colonized by the culture they conquered (which heavily romanized with Celtic substrate) and then later further colonized by norman conquerors who were themselves carriers of the remnants of the Roman cultural heritage.
Germanic people (franks) conquered Gaul and you wouldn't call modern french a Germanic language.
Linguistic dynamics are utterly fascinating and complex
I've been listening to The Rest is History podcast lately, and a lot of this happened in Islam via converts.
Tom Holland was saying that many now fundamental Islamic practices were imported into the faith via converts. For example such as praying 5 times a day was apparently a Zoroastrian practice.
The same thing happened to many Cristian practices.
The Eucharist may have originated in the cult of Dionysus.
Probably also Christmas is also a tradition that bears the roots of a pre-christian festivity that has been merged with / subsumed in Christian tradition
Germanic people (franks) conquered Gaul and you wouldn't call modern french a Germanic language.
Linguistic dynamics are utterly fascinating and complex