What is the significance of language in essay writing?

 What is the significance of language in essay writing? If a student is weak in English, what kind of reading should he/she do to improve his/her language proficiency?

The language should be simple but effective. Xth standard English is more than sufficient to put thoughts effectively. Queen's English is not required.

This is not an English test but it is a communication test. Whatever the student is writing should be understood by any reader, and this is a work place requirement. For e.g., everything moves on files in govt. and whatever is noted on file should be understood by subordinates, peer, seniors, etc. 

To increase effectiveness in writing, easy and interesting texts should be read such as school story books, English versions of comics and newspapers.