General understanding:
* Comprehensible
* Understandable
* Clear
* Plain
* Lucid
* Coherent
* Articulate
* Expressive
For spoken language:
* Audible
* Distinct
* Well-spoken
For written language:
* Readable
* Clear
* Concise
* Fluent
For ideas or concepts:
* Grasppable
* Cogent
* Rational
* Logical
The best synonym will depend on the specific meaning you're trying to convey.