Learning a foreign language takes time and dedication. The reasons below may help to convince you to take the plunge, if such persuasion is needed. Some reasons are practical, some aspirational, some intellectual and others sentimental, but whatever your reasons, having a clear idea of why you're learning a language can help to motivate you in your studies.
When you move to a different country or region, learning the local language will help you to communicate and integrate with the local community. Even if many of the locals speak your language, for example if your L1 is English and you move to the Netherlands, it's still worth your while learning the local language. Doing so will demonstrate your interest in and commitment to the new country.
Family and friends
If your partner, in-laws, relatives or friends speak a different language, learning that language will help you to communicate with them. It can also give you a better understanding of their culture and way of thinking.
Work
If your work involves regular contact with speakers of foreign languages, being able to talk to them in their own languages will help you to communicate with them. It may also help you to make sales and to negotiate and secure contracts. Knowledge of foreign languages may also increase your chances of finding a new job, getting a promotion or a transfer overseas, or of going on foreign business trips.
Many English-speaking business people don't bother to learn other languages because they believe that most of the people they do business with in foreign countries can speak English, and if they don't speak English, interpreters can be used. The lack of foreign language knowledge puts the English speakers at a disadvantage. In meetings, for example, the people on the other side can discuss things amongst themselves in their own language without the English speakers understanding, and using interpreters slows everything down. In any socialising after the meetings the locals will probably feel more comfortable using their own language rather than English.
Study or research
Learning other languages gives you access to a greater range of information about your subject and enables you to communicate with students and researchers from other countries. If much of the information and research about a subject that interests you is in other languages, learning those languages will be very useful. For example, if you're interested in minority languages of Brazil, you will probably find most information about them is in Portuguese.
Many English speakers seem to believe that wherever you go on holiday you can get by speaking English, so there's no point in learning any other languages. If people don't understand you all you have to do is speak slowly and turn up the volume. You can more or less get away with this, as long as you stick to popular tourist resorts and hotels where you can usually find someone who speaks English. However, if you want to venture beyond such places, to get to know the locals, to read signs, menus, etc, knowing the local language is very useful.
Americans who travel abroad for the first time are often shocked to discover that, despite all the progress that has been made in the last 30 years, many foreign people still speak in foreign languages.
(Dave Barry)