extract if_index without grep from scutil --dns
If you look at the executable itself, /usr/sbin/scutil with the strings command you can see that those strings are hardcoded directly into the executable.

$ strings /usr/sbin/scutil | grep 'if_index'
  if_index : %d (%s)


Additionally this CLI's output remains the same when you try changing the LANG=.. or LC_ALL=.. environment variables. For example:

$ LANG="zh_CN.UTF-8" scutil --dns | grep 'if_index'
  if_index : 10 (en0)

$ LC_ALL="zh_CN.UTF-8" scutil --dns | grep 'if_index'
  if_index : 10 (en0)


Doing this with another CLI tool such as git translates the output:

$ LC_ALL="zh_CN.UTF-8" git --help
用法：git [--version] [--help] [-C &lt;路径&gt;] [-c &lt;名称&gt;=&lt;取值&gt;]
           [--exec-path[=&lt;路径&gt;]] [--html-path] [--man-path] [--info-path]
           [-p | --paginate | -P | --no-pager] [--no-replace-objects] [--bare]
           [--git-dir=&lt;路径&gt;] [--work-tree=&lt;路径&gt;] [--namespace=&lt;名称&gt;]
           &lt;命令&gt; [&lt;参数&gt;]

这些是各种场合常见的 Git 命令：

开始一个工作区（参见：git help tutorial）
   clone      克隆一个仓库到一个新目录
   init       创建一个空的 Git 仓库或重新初始化一个已存在的仓库


All this leads me to believe that the output will not change.