Firstly, Let me tell you very clearly, SEO is all about search engines and a website ranking position on the search engine like Google and you have work in the English language which requires average skills for reading and writing in English to optimize content, keywords, and tags.
If you ask me about my opinion by SEO Point of view then I would say no, No you don’t need a very good level of English skills to perform SEO Activities but the scenario will not be the same when it comes to getting a job in an established company for as an SEO Specialist.

The requirement of English in Small Companies for SEO Job:

Getting a job in Small company for SEO Person is easy even if you don’t have good English skill. You only have to know the right techniques for performing well on the search engine. They will not conduct any HR (Human Resource) Interview session to check your speaking, behaving, personality or communication skills but will check your confidence that you can perform at your job and contribute as a big part of the company. It will be more good if you have other skills and overcome the “English” Factor like:

The problem-solving skill that you can come in front when a problem occurs.
Confidence is “yes, I can do that” and talking about the problem with techniques using by your company.
Talk and have a clear conversation in your comfortable language.
Understanding of Tools.
Optimize Keywords in content and maintain a good density of keyword.

The requirement of English in Big Companies for SEO Job:

If you don’t have a good level of English skills then it might be a problem for you to get a job in well-established companies because they hire a person by considering many factors such as Personality, Communication Skills, Past Work Experience with a detailed report on previously ranked website of search engine. You sometimes also need to talk to your seniors in English for any kind problem which also makes your survival hard in the company. Then how will you get a stable job in a good company with an average level of English skills and stay in that company longer period of duration:

Try to learn English for interview purpose only because it is the only day you need this skill.
Being confident in all aspect of your work profile.
Summarize “Tell about yourself” session in English and memorize it.

Should You Learn English for SEO Job?

The whole point is you don’t have to learn English to do SEO or performing SEO techniques but you need to learn some good English skills to get a job in well-established companies