How to create multilingual (English, Japanese) html document with htlatex

You can use one of helpers4ht packages, alternative4ht. It isn't on CTAN yet, but installation is easy.

This method is described in a tutorial, your document needs only small changes:

\documentclass{article}
\usepackage{import}
\usepackage{alternative4ht}
%\nonstopmode
\usepackage{makeidx}
\usepackage{appendix}
\usepackage{verbatim}
\usepackage{graphicx}
\usepackage[table]{xcolor}
\usepackage{enumerate}
\usepackage{longtable}
\usepackage{multirow}
\usepackage{hyperref}
%
\altusepackage{fontspec}
\altusepackage{xeCJK}
%\usepackage[TS1,T1]{fontenc}
\altusepackage{xunicode}
\setCJKmainfont{AozoraMinchoRegular}
\defaultfontfeatures{Mapping=tex-text} % To support LaTeX quoting style
\defaultfontfeatures{Ligatures=TeX}
\pagenumbering{roman}
\providecommand{\tabularnewline}{\\}
\newcommand{\documentname}{マニュアル}
\title{TestHTML}
\date{\mydate{\today}}
\begin{document}
\clearpage

% -------- sections ----------------
\pagenumbering{arabic}
\setcounter{page}{1}

% end preamble
% Chapters
\documentname この章では、コマンドラインインターフェースで使用可能な機能について解説します。ユーザが使用できるオプションについて知り>たい場合は本章>を参照してください。
This section is in english.
\end{document}

important lines are

\usepackage{alternative4ht}
...
\altusepackage{fontspec}
\altusepackage{xeCJK}
%\usepackage[TS1,T1]{fontenc}
\altusepackage{xunicode}

this will load special version of these packages, which doesn't fail under tex4ht. The config.cfg can look like:

\Preamble{html}
\begin{document}
\Css{
body {
  background: url(doc_preamble_html.jpg) no-repeat center center fixed;
  background-repeat:no-repeat;
  font-family:"ヒラギノ角ゴ Pro W3", "Hiragino Kaku Gothic Pro",Osaka, "メイリオ", Meiryo, "ＭＳ Ｐゴシック", "MS PGothic", sans-serif;
  background-attachment:fixed;
  background-position:top;
}
}
\EndPreamble

The file needs to be compiled with lualatex as compiler, which can be done using:

make4ht -ulc config sample.tex

make4ht is alternative build system for tex4ht, it has much more features than htlatex.

The result: