Think of just something simple like writing an immigrants name inside a sentence. It's kinda funny that people in SV, full of immigrants, never seem to think of putting their own or coworkers name in a String.

I'm not a linguist and that will probably be readily apparent. The word jalapeño leaves me wondering how distinct a boundary a language can possess or how one can sort out which language an individual word belongs to outside the context of the rest of the text or speech.

In English, jalapeño is correctly spelled with or without the eñe (and AFAIK the letter doesn't have a name in English, you have to use the Spanish name). So, there's an English word that doesn't use the letters assigned to the English alphabet. How do we place the word? Well, obviously English borrowed the word from Spanish, so it's a Spanish word. Well, no, it's only the Spanish adjectivization of Nahautl words used to name the place called Xalapa...

Words like angst or ersatz are English words borrowed from German. The German words are written identically (except capitalisation), but the meaning of the English word is much more specific than the German "original". Meanwhile the word "Blitz" has completely district meanings in English and German. In English it's a sudden concerted effort, in German it's lightning. Despite the English word originating from German, they don't share a meaning at all

Isn't the etymology of 'blitz' in English due more to 'blitzkrieg'?

German has the word blitzschnell which means really fast (as fast a bolt). So in a way the english meaning of blitz still fits your description.

My grandfather's thesis was auf Deutsch and is sprinkled with French and Latin words.

It’s very common in the online messaging I’ve seen in English, Spanish, Chinese, and Russian.