Why do depictions of foreigners in English media compulsively insert foreign words from their mother tongue?

There is something that has been bugging me about depiction of foreigners in various English media (that doesn't occur, say, in Polish media).

The "foreigner" characters keep replacing common English words they clearly know with common words from their mother tongue in regular, relaxed, every day situations.
As far as I'm aware, this is unlike real people speaking a secondary language, who use words from their mother tongue when they're fishing for a word they don't know or can't remember, or are reflexively cursing. (Or, like Bitter dreggs pointed out, are under stress.)

For example:

• I recall seeing several times, in different TV series or movies, a Mexican-maid-working-in-USA that constantly replaces "yes" with "si" and "sir" with "senior".
• In Brandon Sanderson's novel Elantris, a secondary character that is a foreigner relative to the rest of the cast, keeps using words for "friend" and "understand" in in-fiction language.
• In video game CrossCode, a promiment French character keeps using French phrases that are definitely not common loan phrases in English, and also replaces "my" with "mon".
• And there several more examples that I can never clearly recall when I'm talking about this.

Is this a lazy writing technique that got popularised by some specific work of fiction? Is there some historical precedent where a group of immigrants to England/USA did speak exactly like that?

Solution 1:

There is something that has been bugging me about depiction of foreigners in various English media (that doesn't occur, say, in Polish media).

The "foreigner" characters keep replacing common English words they clearly know with common words from their mother tongue in regular, relaxed, every day situations.

This is relatively easily explained: English literary tradition (and real life) does the converse: It has English speaking characters (and real people) mangling foreign languages by inserting at random the few words of a foreign language that they know:

[To a Frenchman]: "Understandez-vous que je means?" (Do you understand what I mean?)

[To a German]: "Guten Morgen, sell you the cigarettes?"

[To the Spaniard]: "Two cervezas please"

in which the archetypal Englishman will insert foreign words in order to help the foreigner.

This is done on the basis of the accepted truth in the native English-speaking world that "All foreigners speak English, they just pretend that they don't to annoy us."

Thus the "foreigner" characters keep replacing common English words they clearly know with common words from their mother tongue because that is what English speaking people do when addressing foreigners.