Language difficulty is relative. Languages evolved in families determined by geography and migration, and the further a language is on the family tree from your native tongue, the harder you will find it to learn. The Defense Language Institute, which trains government employees at its Foreign Language Center, lists Arabic, Chinese Mandarin, Korean, Japanese and Pashto as "Category IV," the languages that will take the longest for English speakers to master. But the DLI only teaches 24 of the hundreds of languages on this planet. It's likely that learning to speak a click language or getting fluent in Basque is even harder.
Khoisan: Communicating With Clicks
The Khoisan languages were once spoken across the southern half of the African continent. Khoisan speakers communicate using a variety of clicking sounds. There are four basic clicks, but the exact way in which they are articulated changes the meaning, and they combine with other sounds; one Khoisan language has 126 different consonants. Word and sentence structure are also completely foreign to the ear of the English speaker. According to the About World Languages website, no data describe how hard Khoisan languages are for English speakers to learn, but no other language has a sound structure like this that is utterly foreign to English speakers.
Basque: The Isolate
According to language website Lexophiles, when the British government surveyed diplomats everywhere about the hardest language to learn, there was a clear winner: Basque. Lexophile's own user poll came to the same conclusion. The Basque region straddles the border of France and Spain and has its own distinct culture. Basque mystifies linguists because it's an "isolate," with no roots that can be traced to other languages. Basque has five different dialects, and neither the vocabulary nor the sentence structure has anything in common with English.
Hungarian: Vowel Harmony
The second choice of the surveyed diplomats, the language of Hungary, features 17 distinct noun cases, 16 ways of conjugating a verb and 40 letters in its alphabet. Of those 40 letters, some are made up of two or three parts. There are "special consonants" and an entire concept called "vowel harmony" that must be mastered. The Defense Language Institute doesn't teach Hungarian.
Mandarin Chinese: All About Tone
Many people consider Mandarin Chinese to be among the hardest languages to master. This is certainly true of the written language, which is made up of 80,000 characters. The spoken language is tonal, which means that the tone in which a sound is pronounced changes the entire meaning of the word. Four distinct tones need to be mastered before you can reliably be understood. It's said that people with an ear for music have an easier time learning Mandarin Chinese.