Formal:
* Monolingual: This is the most direct opposite, meaning speaking only one language.
* Unilingual: Similar to monolingual, but slightly less common.
Informal:
* Unilingual: Used similarly to the formal version.
* Single-lingual: Emphasizing the "one language" aspect.
* Non-bilingual: Directly negating the meaning of bilingual.
Contextual:
* Unskilled in another language: This is more descriptive of a lack of fluency in another language, rather than simply not being bilingual.
* Limited in language ability: This describes someone who may have some understanding of another language, but not fluent enough to be considered bilingual.
Ultimately, the best antonym for "bilingual" depends on the specific context and the nuance you want to convey.