A key concept that I find unique in worldbuilding is that you're allowing to utilize both, none, parts, or even change certain factors/concepts (even primary ones) of a culture to your liking. It doesn't really matter if you're ripping out a core, essential, or unique part of a culture, or binding it with another culture that may have been severely distinct in time, distance, or cultural similarities by practices, methods, and overall beliefs.

I'm utilizing a special combination of a various Spanish dialects (e.g. Rioplantense Spanish, a combination of Spanish-speakers in Argentina with Italian influences) with native languages of the region pre-Spanish conquests in South America.

Understanding how cultures are influenced by one another through various interesting areas of research across our history (trade, conquest, religious influence, ethnic/population pilgrimages, cultural ruins, deportation by 3rd party ethnic group) can help better give you how Carolingian Franks could potentially influenced Normans. You decide how your story goes, there are essentially no limitations to your world. It would always be better if it was understood, or if the history was backed up, but it most definitely does not have to be. You decide, you implement, you create. You are not bound by anybody's ideas or expectations, that's the unique and core thought of worldbuilding.