Having the ability to communicate in more than one language is known to be beneficial and an attractive quality for a long time. Not only does it increase your marketability as an individual for job opportunities, as a student, etc., but being multilingual is known to help improve soft-skills such as multitasking.
While this is good in theory, realistically learning a language is quite a difficult task to accomplish, especially as an adult. Each learner is different and the time it takes to learn a new language depends on multiple factors such as the complexity of the target language, the time you can devote to learning it, the resources available to you, how similar it is to the language you are already fluent in, and ultimately, your motivation to learn.
Here we list a number of languages from easiest to hardest for English speakers to pick up depending on the similarity to the English language and the time frame of achieving language proficiency.
The majority of these languages are Western European languages and include Spanish, French, Romanian, Swedish, Norwegian, Portuguese, Italian, Dutch, German, Danish, and Afrikaans. In general, native English speakers take between 23 to 30 weeks (575 to 750 classroom hours) to gain proficiency in these languages.
These languages include Hindi, Russian, Vietnamese, Turkish, Polish, Thai, Serbian, Greek, Finnish, Hebrew, and Persian. Because of there linguistic and cultural difference, these languages take more time for English speakers to learn. The time frame could be anywhere between 36 to 44 weeks, and 900 to 1,100 classroom hours.
These five languages are known to be exceptionally difficult for English speakers to learn, and one of the factors that contributes to this are their completely different writing systems. These languages are Arabic (all varieties), Cantonese, Japanese, Korean, and Mandarin. In order to reach proficiency in these languages you will require at least 88 weeks and 2,200 class hours of study in general plus a second year of study in the foreign country.
Learning some of these languages may seem pretty daunting, however, don't let that deter you from learning a more "difficult" language. It's very important to remember that language difficulty is relative, and what may take one learner 3 weeks to learn could be learned in a few days by another learner.