"To write the language" means to be proficient in expressing oneself through written words in a specific language. It refers to the ability to write effectively and communicate effectively through writing. Full definition
Without diagnosis, children may not receive early intervention or specialized instruction in all the relevant skills that are interfering with their learning of written language. (dyslexiaida.org)
The admission essay is an important part of your application, but many students who aren't comfortable expressing their thoughts and feelings in written language struggle with it. (onlinecollegeessay.com)
Hence, most of them struggle with written language as well as with text reading and comprehension. (sciencedaily.com)