英語が不得意で申し訳ないのですがって英語でなんて言うの？

By saying "I apologize for.." or "Please excuse me because..." when you want to ask for not doing something well or for making some kind of mistake. It makes the apology more sincere and polite.

Well, it is probably sufficient simply to draw the reader's attention to your level of English at the beginning. Probably an apology is not necessary. We cannot all be equally brilliant at everything!

You can say " I am not proficient in English" or "I would like to apologize for my poor English."

The recipient will understand by that context that you are not fluent in English.
When you say "I apologize" it means that you are sorry.

"not proficient" would mean unskilled.

You can also say "English is not my mother tongue so I apologize for my poor writing."

You can mention that you are still learning English, that will help someone understand why you may not clearly understand some words or phrases.
If you are fluent, it means you fully understand everything. If you are still learning English, you could say that you are not fluent.

I'm sorry would be a simple way to apologise, Then explaining that your English is not very good or strong is a good way of saying that your English is poor.

If you want to apologize in a business setting we would say "Sorry for my English ability" or "Sorry for any mistakes". These will let the person you are talking to know that you aren't completely fluent in English and that it is not your first language. Most people won't be upset. You can also say that you are happy to answer any questions or concern they have about your English or email message. "Please excuse my English" is an extremely polite way to say that you are sorry for your English ability.

to be formal you could say something like "I apologize in advance for my poor english"
or to speak more casually you could say something like "I'm sorry but my english is not very good"

"I'd like to apologize for my English skills"
To 'Apologize' for something is to say sorry, you are sorry for, in this sentence, you English skills, this implies the English skills may not be very good if someone if apologizing for them.
"Please excuse my poor English"
To 'Excuse' something is another term used as apologizing for something.
If something is 'Poor' it is often meant that it is not very good/the level isn't very high.

When you would like to alert the reader of your email in advance, that your English skills are poor, you may use the sentences above.

Unpolished: rough, not perfect
Underdeveloped: not correctly developed, not complete
Non-fluent: unable to write or speak correctly in a given language
Minimal: little

"Please keep in mind that my English fluency is minimal."

Saying this just lets the person know you are sorry that your English isn’t the best before starting your email.

Ex: -“I want to apologize ahead of time for my poor English.”

-“I want to say sorry in advance for my poor English.”