Learning a second language is not the same learning a first language.

Применим протокол К.

Подставим каждый вариант в предложение.

Learning a second language is not the same as learning a first language. - подходит по смыслу.

Learning a second language is not the same than learning a first language. - не подходит по смыслу.

Learning a second language is not the same like learning a first language. - не подходит по смыслу.

Learning a second language is not the same then learning a first language. - не подходит по смыслу.

Запишем корректный вариант.

Learning a second language is not the same as learning a first language.

Ответ: Learning a second language is not the same as learning a first language.