There can be some confusion when we ask about how to describe a "sentence", and then discuss properties of how the sentence is written down. We should distinguish language from writing ; they are distinct things, with independent properties. Writing is (basically) a representation of language.
Let's think of the language first. The key question is, what linguistic utterance is that piece of writing representing?
- If it's representing "I live in Měiguó" (or "I live in Mei⁵gwok³", etc.), then the sentence displays code-switching, or borrowing.
- Macaronic is a somewhat more informal term for frequent code-switching, sometimes mixing parts of words. I wouldn't say this sentence looks particularly macaronic, though it could be part of a larger macaronic rant.
- If it's representing "I live in America"—that is, if the characters ‹美国› are supposed to be read as "America", the way e.g. Japanese read ‹大人› as otona—then there's no code-switching or borrowing going on. It's just an everyday English sentence.
Now we turn to writing. ‹I live in 美国› is a mixture of two writing systems. I believe "mixed orthography" or "mixed writing" are reasonable labels for this phenomenon. This is independent of how many languages are being represented; for example, ‹I live in Америкa› is mixed writing representing a pure English sentence.