The past is a foreign country; it is also a foreign language.

The line plays on a well-known phrasing about the past being "a foreign country" and extends it by adding that it can also be thought of as a "foreign language," emphasizing how difficult it can be to interpret earlier times without learning their meanings and conventions.

Understanding history requires translation: people in earlier periods used different assumptions, values, and ways of expressing ideas, so the past can feel distant and hard to read without careful interpretation.