Many people who have been US Permanent Residents for a very long time and qualify for Naturalization are reluctant to apply for various reasons, including language issues. They may feel secure because they have had their green cards for a long time and have never had problems.
However, the fact is that while Permanent Residence is certainly a desirable thing, the ultimate goal should be attaining citizenship, if possible.
The reason is that the name Permanent Residence may be somewhat misleading in that the status may in fact turn out not to be permanent. That is, a person may loose his or her permanent residence for many reasons, including abandonement by staying out of the country for long periods of time, or having committed certain acts that may lead to loosing a person's residence.
So , if you qualify, you should consider applying for Naturalization and, if fear of language is holding you back, you should make every effort to gain proficiency in the English language or checking with an immigration attorney, or USCIS, to see whether you qualify for any of the existing exceptions regarding language.