僕は英語が話せません。凄く簡単な単語がわかるていどですって英語でなんて言うの？

"My English is not great." is a simple way of letting others know that English is not your first language, or that you are currently learning English.

"I can only understand basic words in English." Helps people to know that your English vocabulary is limited.

I find that body language and a smile answers this best. However if you want to use words I find that phrase "I'm sorry I don't speak much English" or "I only know (very) basic English" to be a good polite way of telling people not to engage in a conversation with you.

If you have no time then the statement "no English" also works whilst smiling!

If you have barely started something, it means that you have only very recently started that thing.
"I was barely 10 years old when my parents separated."

"No English"

In reality if you do not speak English at all and only understand a few basic words than you are probably better off at saying this expression. Explain that you only understand a few basic words is probably going to be very hard for you so if you use this expression in the U.S. most people will understand that you do not speak English.

When you want to explain that your understanding of the English language is not very good but you can only understand very simple words, then you may say it in the following ways:
-My English is not very good. I can only understand very simple words.
-I'm not proficient in English. I can only understand simple/basic words.

You can say, "I am not fluent," to show that you are still learning to speak English fluently. This can signify to them to slow down or simplify their speech. If you request, "Only simple English please," they will know to use easy, basic English with you. Lastly, you can tell them, "I am still learning English," so they will understand if there is a miscommunication.