The Foreign Services Institute developed their language courses to help diplomats quickly reach professional working proficiency in a language. They would attend 5 hours of language instruction per day, plus homework, for 24-88 weeks (depending on the language). The table at the bottom of this page indicates the average time it would take for a student to reach professional working proficiency.
The FSI program places a strong focus on listening comprehension, in addition to extensive exercises for grammar and vocabulary. If you follow along with the audio and respond to the prompts in each drill, you will also develop confidence in speaking the language.
With hundreds of pages of text, dozens of hours of audio, and several levels in many languages, the FSI courses are still probably the most comprehensive, free courses you can follow. Just remember that they are decades old, so the audio is not very clear and the vocabulary often includes both sexist and obsolete language. Also, some of the topics will not be relevant to your everyday life.
Several sites host these free, open-source courses, but the site linked below is easy to navigate. Beware of any site selling courses “originally made for diplomats,” as these are probably free FSI courses with a price tag. You can also check out the DLI courses, which are equally as comprehensive (and free!).
The current rating is our best estimate. We haven’t had the opportunity yet to more thoroughly evaluate this resource, as we do for our full reviews.