About [[MediaWiki:Createacct-imgcaptcha-help]]

The same applies to the following message:

• Createacct-helpusername-link ("[[{{MediaWiki:createacct-helpusername-url}}|(help me choose)]]")

No, it's a link defined locally so it must use the wiki's language.

You mean not language from my preferences but content language of wiki all the time?

Exactly; the message contains the title of a page on the wiki, (and e.g. German Wiktionary is hardly going to have its policies translated to every language on Earth) so it needs to be fixed to the content language of the wiki.