I also use Japanese-letters, so I know that this behavior is frustrating..

I opened #6385 but I don't think we should add new option for this now. See #6385 (comment). I agree the comment. Prettier should simply keep whitespace same as the input.
However, it is a big breaking change so we cannot add this change in minor and patch release. I think We can do it in next major release.

/cc @ikatyang How do you think?

@sosukesuzuki I think the main problem with what I wrote there is line breaks. If --prose-wrap is set to always or never, Prettier needs to re-wrap text. If Latin and Japanese or Chinese meet on the line border:

.... .... .... .... ... .. ......... ....... <CJK>
<LATIN> .... .... ...... .. ...... .... ..

what should Prettier replace the line break with? A space or the empty string?

Because most plugins for removing linebreaks between CJK (e.g. markdown-it-cjk-breaks, hexo-filter-fix-cjk-spacing) don't treat linebreaks between CJK and non-CJK, in my opinion, it seems like a good idea to follow them and to leave a space:

Input

あおえ
うい
aoe
ui

Output

あおえうい aoe ui

The reason why I focused on the behavior of plugins is the release note.

Note for CJK users: If your markdown renderer does not support CJK line ending, you'll have to use plugin like markdown-it-perfect-newline-for-cjk, hexo-filter-fix-cjk-spacing, etc. to remove additional spaces.

// Source
一二三
四五六
七八九

// Rendered content with unsupported renderer
一二三 四五六 七八九

// Rendered content with supported renderer or via plugin
一二三四五六七八九

Thus, we warn users that formatting CJK text with --prose-wrap always could output undesirable results.

Input 1

私はJavaScriptが好きです。

Output 1 (prettier --prose-wrap always) / Input 2

私はJavaScript
が好きです。

Output 2 (prettier --prose-wrap never or markdown parsers with plugins)

私はJavaScript が好きです。

I really want this option merged into prettier and released, because I rely on prettier heavily.
Now I have to ignore the .md files while formatting.

@uptonking We know this is a problem, but we'd like to try to solve it without adding an option.

@thorn0
As for Chinese language, it's quite a personal taste about whether to add a whitespace between Chinese and English characters.
Sometimes it's better, sometimes not. Some people prefer, some people not.
Someone even writes a guide for the recommended format for Chinese as stated in issue 6385. But I don't agree and won't adopt it.
So I strongly request an option. Otherwise github issues will always be open.

@uptonking no options, we will keep original input as it, if you need ensure one style on all docs, you need to use linter

I agree with evilebottnawi. As I mentioned on Twitter it is NOT Prettier role.

Sounds good. I am relieved to know that the discussion is continuing. Now I will close this PR because the PR is different from those plans.

It looks to me from #7933 (comment) like this change to disable adding spaces is wanted but with no option in the next major release. Considering that these changes seem to simplify src/language-markdown/utils.js, should this PR be resurrected with the option removed?