How much would non-native speaker's English mistakes affect conversion?

I think it could have a rather large effect, I would get a proofreader to help.

I think it is going to vary from reader to reader and buyer to buyer.

Some people want to see perfect English and expect a product to have been proofread so that it is mistake free (or nearly so).

Others will forgive an occasional mistake here and there. No one is perfect, right?

Then there are those that will say . . . if the quality is in the product, I can forgive the English writing mistakes.

Personally, I believe that your business should take pride in paying attention to the little details. Mistakes suggest sloppiness and a make one wonder if this same trend will carry over into the products from this business.

I would suggest paying a proofreader to check your sales letter, your product, your web site copy, etc. Make these things as mistake-free as possible. There shouldn't be a huge expense for proofreading.

When it comes to blog posts, emails, etc, (the more personal communications) to me at least, I am not so picky because I understand that bloggers come from varying backgrounds and they are speaking in their natural everyday voice where some mistakes can be expected.

However, when mistakes are rampant, and every sentence has one or more, it gets to be irritating. The numerous mistakes take one's attention away from the content because one looks for the next mistake.

Just my thinking,

Steve

Stick with Japanese (your native language). Language mistakes will cost anyone dearly in terms of conversions.

Thank you both for your replies.

Especially Steve, you cleared up a lot of my questions.
Yes I was going to have my product, sales letter, main website pages etc. proofread anyway. I was more worried about blog posts (articles) and emails because if I wwere to have them proofread too then that would be endless, and also it'll take time before I can actually publish/send it after I write them.

I think I'm going to not worry so much about blog posts or emails, but I'll do my best to "perfect" the product and the "formal part" of my website.

You can either hire a proofreader or completely outsource to an English writer if you want perfect writing.

But then again, I'm a native speaker, and suck in every other language.... And I don't even do English very much justice lol.

Occasional mistakes are fine, but it largely depends upon your target demo. You already received good inputs from others.

I've never used grammar-checking software before, other than that which is included in word processors and other text editing software, but I would think that a program like the one in the link below might be of great help in eliminating most of those "little" errors that you might make.

https://www.grammarly.com

If they got your point, why would they care about the grammar mistakes?

It depends, as long as your grammar won't change the definition or the things you really want them to know. If they need what you offer, nobody gives a s**t about your grammar.

I am impressed with your excellent English writing. Just skimming I only saw a few minor mistakes.

You have a good plan. Hire a well-qualified proof reader for the most important "first impression" pages. You might even prepare your readers by letting them know English is not your first language and there may be some occasional slips.

Depending on your niche and your products most people are more interested in authenticity and the value you bring. I would rather read awesome content with real value that had a few grammatical errors than perfectly written fluff.

It does matter. If i have to put up trying to piece things together on your sales page... then i know for a fact that i'm going to have to do the same thing when i get your product. This can be something that can cause a ton of refunds. Create your product, then have someone fluent in English edit and proofread it for you.