This free online software tool calculates readability :

The measure of readability used here is the indication of number of years of (American) education that a person needs to be able to understand the text easily on the first reading.

In general, these tests penalize writers for polysyllabic words and long, complex sentences. Your writing will score better when you: use simpler diction, write short sentences. It also displays complicated sentences (with many words and syllables) with suggestions for what you might do to improve its readability.

This tool is made primarily for English texts.

https://www.online-utility.org/english/readability_test_and_improve.jsp