スペース区切りかもしれない文字列のアクロニムを3文字ぶん切り出す(u)pbibtex関数（ただし漢字を含む文字列の場合は先頭の2文字を切り出す）

acronym.bst

STRINGS { s t str org car result }
INTEGERS { len }

FUNCTION {chop.word}
{ 's :=
  'len :=
  s #1 len substring$ =
    { s len #1 + global.max$ substring$ }
    { s }
  if$
}

FUNCTION {chop.article}
{ 't :=
  "A" " " * #2
    "An" " " * #3
      "The" " " * #4 t chop.word
    chop.word
  chop.word
}

FUNCTION {acronym.word}
{ duplicate$ 'org := chop.article 'str := 
  org is.kanji.str$ 
    { org #5 text.prefix$ purify$ 'result := }
    { str #1 #1 substring$ 'result := 
      { str empty$ #0 = }
      { str #1 #1 substring$ 'car := 
        car " " = 
          { str #2 global.max$ substring$ chop.article 'str :=
            result str #1 #1 substring$ * 'result := 
          } { skip$ } if$
        str #2 global.max$ substring$ 'str :=
      }
      while$
      result text.length$ #3 <
        { org chop.article #1 #3 substring$ 'result := }
        { result #1 #3 substring$ 'result := }
    if$
    }
  if$
  result
}

FUNCTION {test}
{ "abc dfg df fg" acronym.word top$ }

FUNCTION {jatest}
{ "日本語が   含まれる場合" acronym.word top$ }

ENTRY{}{}{}

READ
EXECUTE{test}   % add
EXECUTE{jatest} % 日本

bibtest.tex

\documentclass[uplatex,dvipdfmx]{jbook}
\begin{document}
\bibliographystyle{acronym}
\bibliography{anything.bib}
\end{document}

exec.sh

$ upbibtex bibtest
This is upBibTeX, Version 0.99d-j0.33-u1.26 (utf8.uptex) (TeX Live 2020)
The top-level auxiliary file: bibtest.aux
The style file: acronym.bst
I found no \citation commands---while reading file bibtest.aux
Warning--I didn't find any fields--line 51 of file acronym.bst
Database file #1: anything.bib
add
日本
(There were 1 error messages)
$