"无法理解的语言" can be translated into English as "an incomprehensible language" or "a language that defies comprehension".
Both expressions convey the idea of a language that is difficult or impossible to understand. The first one, "an incomprehensible language," is more straightforward and commonly used, while the second one, "a language that defies comprehension," adds a bit more emphasis on the difficulty of understanding.
Choose the one that best fits your context or preference.