My high school students often ask me which foreign language is the easiest to learn. Of course, every language has its pros and cons. But before deciding on a language to study, consider the amount of time it typically takes a native English speaker to achieve proficiency.
Fortunately, the U.S. Department of State has classified languages into three categories to make your choice a little easier. These categories are used by the Foreign Service Institute (FSI) for training American diplomats and other professionals from the State Department and the military. The George P. Shultz National Foreign Affairs Training Center in Virginia offers courses in approximately 70 world languages. The following list is by no means complete, but it will give you an idea of what it takes to learn various languages.
Category I
These languages are closely related to English. 575-600 class hours are necessary for proficiency.
Category I ½
Though not an official category, it is the logical place to mention German (750 class hours) and Indonesian (900 class hours)
Category II
These languages vary significantly from English, requiring 1100 class hours for proficiency.
Category III
Plan to study at least 2200 hours to learn these difficult languages. The FSI also requires a second year of study in the target culture.
Ultimately, it is your dedication and motivation that will determine your success. Choose a language and culture that interests you, and work hard for as long as it takes to achieve proficiency.
Sources: National Virtual Translation Center website, Foreign Service Institute website