Why learn a language?

Kolik jazyků znáš, tolikrát jsi člověkem.
You live a new life for every new language you speak.
If you know only one language, you live only once.
(Czech proverb)

Language is the archives of history.
(Ralph Waldo Emerson)

Die Grenzen meiner Sprache bedeuten die Grenzen meiner Welt.
The limits of my language are the limits of my universe.
(Ludwig Wittgenstein)

Most people in the world are multilingual, and everybody could be; no one is rigorously excluded from another's language community except through lack of time and effort. Different languages protect and nourish the growth of different cultures, where different pathways of human knowledge can be discovered. They certainly make life richer for those who know more than one of them.
(Nicholas Ostler, Empires of the Word)

Wer fremde Sprachen nicht kennt, weiß nichts von seiner eigenen
Those who know no foreign language knows nothing of their mother tongue.
(Johann Wolfgang von Goethe)