Diglossia refers to a situation in which two different varieties of a language are used in the same speech community, each serving a different purpose. One variety, typically referred to as the high variety or prestige variety, is used for formal or prestigious situations, such as education, media, and official contexts. The other variety, typically referred to as the low variety or vernacular variety, is used for informal or everyday communication.
Interference, on the other hand, refers to the influence that one language or language variety has on another. It can occur when speakers of one language or language variety learn or use another language or language variety, and the characteristics of their first language or language variety influence the way they speak or write the second language or language variety. Interference can also occur when speakers of different language varieties within the same language use elements of one variety in the other, leading to a blending of the two varieties.