Formal terms:
* Monolingual: This is the most common and accurate term, literally meaning "one language."
* Unilingual: This is a less common synonym for monolingual.
* Single-lingual: This term is less common but still understood.
Informal terms:
* Speaking only one language: This is a straightforward and understandable way to describe the situation.
* Being a one-language speaker: This is another informal way to describe the situation.
The best term to use depends on the context. In formal writing, "monolingual" is the most appropriate. In informal conversation, any of the terms above can be used.
It's also worth noting that the concept of being "monolingual" is becoming increasingly complex. In a globalized world, many people who speak only one language may still be exposed to and understand some elements of other languages. So, while the term "monolingual" is still useful, it's important to consider the nuances of language learning and use in today's society.