Been a while since we had something truly water-cooler in here.... but I ran across this and found it an interesting read: Difficult languages: Tongue twisters | The Economist
English-speakers appreciate this when they try to learn other languages. A Spanish verb has six present-tense forms, and six each in the preterite, imperfect, future, conditional, subjunctive and two different past subjunctives, for a total of 48 forms. German has three genders, seemingly so random that Mark Twain wondered why “a young lady has no sex, but a turnip has”. (Mädchen is neuter, whereas Steckrübe is feminine.)
For sound complexity, one language stands out. !Xóõ, spoken by just a few thousand, mostly in Botswana, has a blistering array of unusual sounds. Its vowels include plain, pharyngealised, strident and breathy, and they carry four tones. It has five basic clicks and 17 accompanying ones. The leading expert on the !Xóõ, Tony Traill, developed a lump on his larynx from learning to make their sounds. Further research showed that adult !Xóõ-speakers had the same lump (children had not developed it yet).