[quote=@Dagger]
<Snipped quote by Andreyich>

That’s an interesting point, and I definitely don’t doubt it. However I also have a couple Latinx friends who do use “Spanglish” in their daily life, so I wonder if there’s some variation across people from different cultures? Living in the southwestern region of America, we have a lot of people from Mexico in my county, and it seems to vary even person to person whether they weave Spanish into their speech or not.
[/quote]
[url=https://en.wikipedia.org/wiki/Code-switching]Code-switching[/url] is a real phenomenon that occurs both for bilingual people and for people speaking different varieties of the same language (as the AAVE example in the Wikipedia link points out). Spanglish is an example of that, though it could also be classed as a creole or pidgin language depending on the linguist you ask and the particular people you're talking about.

Code-switching doesn't happen, as I think [@Andreyich] was pointing out, just for 'flavor,' just to let you know that this character's Russian or French or Japanese or what have you. Real code-switching relies on how both speaker and audience perceive each other and how marked the language/language variety being switched to (aka the 'code' in 'code-switching') is. It generally happens when both speakers are familiar with both codes—somebody fluent in Japanese and English might speak a mixture of the two with their parents who know both or with peers who know both; it's unlikely to happen with strangers who don't know both, i.e. friends who don't know both languages. Intimacy is another influence to a degree; code-switching can be a marker of friendship or solidarity in certain cases, and in environments where code-switching is frowned upon or even just abnormal (and thus likely to create alienation), it's less likely to occur as well. There's a lot more to it, but generally I'd say that in the context of RP, it's important to consider two things: the first being if the audience of the character who is code-switching knows both languages being switched to, the second being how intimate these two speakers are (or are trying to present themselves as being). If those two conditions aren't being met, then generally—there are exceptions, of course—I would avoid it.