The wiki species info is unusually detailed:
It looks like they speak the human language, with some impediments such as limited consonants. If the consonants are created by closing nasal passages, are their consonants limited to soft sounds like M and N? Which other consonants are available, which would be replaced by clicks or other sounds they can make, and which augmented by the vocalizers?Klk'k speech is produced primarily as a combination of vowels and tones from the nasal airways and clicks from the tongues and resonating chamber. Manipulation of the closures to the nasal airways also produce a smaller number of consonants. The tonal nature of each airway is independent, and harmonics have semantic and syntactic meanings in most of the native Klk'k languages. The iconic Klk'k nasal horn takes explicit advantage of the dual-tonal capabilities of the Klk'k. Some human observers have likened Klk'k speech to listening to a pair of Hawaiians having an animated conversation with a pair of Khoisan, but most real xeno-linguists attempt to discourage such simplistic comparisons as more misleading than informative. "Klk'k" is itself an anthropic transliteration of their word for themselves, likewise for Ktah, and Tk'latl, etc. As the Klk'k have proven far more adept at understanding spoken human tongues, than the reverse, even if they cannot produce the full range of human sounds, the anthropic forms have become accepted standards, rather than requiring all humans to utilize translator devices to refer to anything of Klk'k origin. Klk'k living or traveling among humans tend to equip themselves with vocalizing augmentation devices that fill in the missing gaps in their ability to emulate human speech.