In fact, if you take a good look at Introduction to Teaching Chinese as a Foreign Language, this question is not difficult to answer.

Moreover, there is no difference between "second language" and "differences between foreign languages"

Only the concepts of "second language teaching" and "foreign language teaching" are distinguished here.

Second language teaching refers to language teaching activities in the target language countries. For example, foreigners come to China to learn Chinese, or China people go to Britain to learn English, which are all second language teaching.

Foreign language teaching refers to non-native language teaching activities in China. For example, China people learn English in China, and British people learn Chinese in Britain.