The English language has a long and colorful history of appropriating words from other languages—bildungsroman, cliché, bon voyage—to name just a few, but English is an ever-evolving language as well, and these four words from other languages are just what English really needs:
Bueno, from Spanish, literal meaning: good
Bueno is just a good all-around word that can be used in many conversational scenarios, for example: "I'm coming over right after work," she said, and you can reply "Bueno!" meaning, "That sounds good to me!" Or perhaps you want to communicate that you feel good in general, bueno is a great word to choose because it has an upbeat sing-song rhythm that the Germanic-derived "good" lacks.
Lagom, from Swedish, literal meaning: sufficient
Lagom is, at first glance, a short, simple declarative word, but it contains multi-faceted meanings very efficient for English language usage. Modern humans constantly strive for balance in their hustle-bustle existence and this is where lagom is useful. A deeper look at the word reveals it means "just the right amount" or "perfect-simple." That's balance, that's lagom.
Sturmfrei, from German, literal meaning: having the house to oneself
In its native language, this word applies to children, specifically children being allowed the freedom to enjoy unstructured time without the control or supervision of their parents. Overscheduled kids raised by helicopter parents could certainly benefit from more sturmfrei in their lives. Can you imagine the creative forces that could be unleashed with sturmfrei? Plus, it is very fun to say.
Itadakimasu, from Japanese, literal meaning: I receive this food
For the English-speaking family that does not say grace before meals, itadakimasu is a worthy consideration. It conveys respect for the food that is before you, and is said before meals. A moment spent considering the origin of the meal, and respect for the person(s) or prepared the meal and made it possible for you to eat, as well as an acknowledgement of good fortune it is to be able to feed oneself, for all these reasons, itadakimasu would be an excellent word to add to our vocabulary.
These are just four words from other languages that English really needs. Given the wide breadth of languages still spoken in this world, choosing just four words isn't easy. Countless words, and languages for that matter, are being lost from underuse all over the world, including English, so adding new ones is forward-thinking, as well as fait accompli (colloquial American) "done deal."