Can anyone suggest an objective way to measure comprehensibility of a text (to be read aloud)?

All tests of readabiIity including Flesch Kincaid are intended to measure texts as they may be read/understood by native speakers of the language of the text.

I am afraid there might be no "objective" way to measure a (a) translated and (b) legal text. There are several variables impeding objectivity: 1. a defendant - (a) native or non-native speaker of the target language; (b) knowledgeable of law or absolute layperson; (c) knowledgeable of listening techniques, etc. 2. the translated text - quality of translation between legal systems and languages (text perceived as "strange" due to differences between legal systems /difficult to understand/, or the text reads smoothly but does not translate the source law properly /easy to understand but does not make sense, etc.).

And also what would be the purpose of measurement? To show that the defendant should understand because the translated text has a particular comprehensibility score? I don´t know ...

A readability formula is not designed to measure comprehension. Readability formulas are designed for writers who are attempting to reach a certain audience. The nature of legal documents, as Marta Chroma mentions makes comprehension a challenge even for native speakers. The easiest way to measure listening comprehension of this type of text (i.e. jury instructions or warnings) is to have the subject retell what he or she has understood, perhaps in his or her native language. You may want to design a rubric with essential elements of the text, and qualitatively determine to what degree the listener has included those elements in his or her retelling.

If you just want to measure effort, then referring to Grice's conversational maxims and the relevency theory of effort to continuance, you could do what my group did in multi-cultural workshop curricula to generate data. We measured length of time it took to read, how many times the reader had to start over, number of questions asked for clarity, and compared that to the readers fluent in the source language reading the same document before translation. Also, you note whether the reader gave up trying to interpret the text or not and at what point did they give up. Effort equates to work and can be quantitatively measured by time.