steps to reproduce:

Write the email in Japanese and send it.

actual results:

Content-Language: en-US is set.

expected results:

Content-Language: ja, ja-JP should set.

The causes of this issue are:

• Content-Language is implicitly added. It's not easy for the user to control it.
• Content-Language reflects the language of the spell-checking dictionary selected when writing the message.
• We need to select at least one dictionary even if we don't use it.
• TB Japanese includes an English spell-checking dictionary. It cannot be deleted or disabled.
• Japanese spell-checking dictionaries are not on the Add-ons for Thunderbird.

This issue probably also occurs in Chinese, as there are no Chinese spell-checking dictionaries on the Add-ons for Thunderbird.
Such headers can cause confusion against RFC 3282 will.

We discussed in some other bug the header should not be set if the number of spelling errors implies it's wrong.

(In reply to Magnus Melin [:mkmelin] from comment #1)

We discussed in some other bug the header should not be set if the number of spelling errors implies it's wrong.

The idea may work between Western languages, but it doesn't work in this case. This is because the English spell-checking dictionary does not detect spelling errors in Japanese messages.

Well it would say the words are all unrecognized (spelling errors), no?

(In reply to Magnus Melin [:mkmelin] from comment #3)

Well it would say the words are all unrecognized (spelling errors), no?

Let's clarify the definition. I would say:

input words
├ recognized
│ ├ is match: spelling o.k.
│ └ no match: spelling error
└ unrecognized

In this define, Japanese words are all unrecognized by TB spell checker. Not spelling error.