Those tests are entirely based on metrics related to word length, number of syllables per word, sentence length, etc... For modern English text, that produces a not-entirely-horrible estimate of difficulty. The King James Bible uses a lot of short words and short sentences, so it scores low, but many of those words are uncommon and many of the sentence structures are unusual, nuances the tests don't cover. To the extent readability tests are useful at all (arguable at best), they very much aren't useful as applied to old texts.

"dumb down" in the question title sounds pejorative, since it risks implying that non-native users of English are dumb. "Simplify" would be a better.

No matter what you eventually do, I think it'd be helpful if you thought of this not as 'dumbing down' but as 'clarifying'

Shouldn't your writing already be concise?

Do you mind adding some examples? In my experience some non-native English speakers can have difficulties with idioms and metaphors, but deal pretty well with obscure words (which might have a common origin in their language).

For example, you say 'colour-blind' and they might not understand but say Daltonic, and they get it, because 'daltónico' is Spanish for 'colour-blind',

My general rule. When writing something that will be read by those who are not native speakers of English, use only simple English. For example, this is an argument against using passive voice for scientific research papers.

In my opinion, you should extend 'dumbing down' to native speakers just as much.

@GEdgar: Is passive voice not simple English? I work with a lot of native Spanish speakers and they seem to love passive voice. Some have told me that they were taught that technical writing should be in passive voice.

Be careful you don't underestimate the English understanding of an educated Dutch person. In my experience they have been using English at a professional level for many years and may be your equal.

As an aside: your idea that you should be writing text with a difficulty of "college graduate" when writing for college graduates is flat out wrong. Harder to read is not, in any way, a merit. You should strive to make your writing as easy to read as possible regardless of audience. Complex subjects may require more complex writing but even very complicated subjects can be communicated well in surprisingly easy to read language by the best writers.

@GEdgar Your argument against passive voice is a misconception. For instance, in my mother tongue (Italian), passive voice is used quite frequently and people are used to it. Actually, when I started writing scientific papers, I had to force myself in using more the active voice. Moreover, many foreigners are quite used to long and winding sentences and have no difficulties in reading them. For instance, my high-school literature book contained a sentence which was half-page long: really, it started at the beginning of the page and in ended with the period in the middle of the page!

Writing precisely what you want to communicate in simple english is and should be both more challenging and commendable. I claim that communicating the exact same idea using a simpler language is similar to producing mathematic proofs. The simpler the more elegant - and the more intellectual the writer. You have an attitude problem, Sir, if you think simple is dumb. Simple is clever. You are asking, "should I be smarter?" and the answer should be a resounding - Yes!

constructions and vocabulary that no reasonable non-native English speaker could ever be expected to know I was obliged to pass an English exam before I was allowed into the UK. Scored higher than most native speakers. Using English as my first language for 8+ years now, and learning it since the age of 3. So I guess I'm unreasonable, and what you can reasonably expect from professionals that use English actively for their teaching, publishing and collaboration is "My Engrish very well, yes.". You need to adjust your attitude, not writing.

Related: bbc.com/capital/story/….

@GEdgar I thought the recommendation against the passive voice comes from dogmatisation of some misguided style guide (whose authors could not even correctly distinguish between passive and active voice) …

@penelope Not everyone speaks English since the age of 3. For that matter, you would be considered a native speaker.

@WaterMolecule I am from Latin America and agree, we are taught to write in passive voice. But that's because Spanish works well with this, as is basically changing a pronoun . But not English. Trying to speak in passive voice requires putting every sentence upside down, and reads very unnatural.

@user4052054 I've never even seen a native speaker before the age of 15, which is when I got my first native speaker teacher. I remember an incident around the age of 10 when I couldn't remember the word "I" is the first person singular in English and my mother got quite angry at me. I started using English on a daily basis 8 years ago, and before that I rarely was in a situation where English was spoken other than a classroom. I think somebody got their definition of "native speaker" mixed up.

@user4052054 Maybe not everybody starts learning English at the age of three, but I am sure that people who are active academics, researchers and are involved in teaching programmes in English use English actively, if not daily, for many years. I would almost guarantee that such positions are filled with people whose level of English understanding is above the required level, especially in the Netherlands, the European country with most foriegn languages spoken per citizen.