Not true in idiomatic English. Any language other than your 'first' can be called a 'second language'. As a common example of this usage, courses to learn English for non-native speakers are called "English as a Second Language (ESL)" - no matter how many other languages the students already know.
Then I guess only native English speakers can have many "second" language. In many other countries there are 1st foreign language and 2nd foreign language classes in the school. And those books are called "English as a foreign language" (Englisch als Fremdsprache).
