The readability of your website depends on your targeted audience.
In most cases, I recommend keeping everything simple to reach the widest audience. In some cases, you may find it better to use a higher level of English:

There are many tools to help you measure readability on websites. Most of the ones I found don’t work. Here are 2 that do:

Caption: Online text readability tool rates reading grade level and give specific tips to make the text more readable.

They will help you make sure you website is easy to understand. If you are a global company, you are probably targeting people that use English as a second language. In this case there are additional considerations:

For more tips on how to increase conversions globally click here