Epic: Localize MacVim and add translations

This issue is used to serve as an epic to track the ongoing work to localize MacVim.

Messages

• Support Vim message translation (see :help multilang-messages) / :set lang (#1070)
• Add MacVim-specific translations for MacVim-specific messages.
  • Welcome messages (some done in #1071)
  • Modified "file changes" messages
  • Etc
• Messages used by MacVim overall app itself (using NSLocalizedString). E.g. when you are quitting using Cmd-Q but have modified files.

Menus

• Support Vim menu translations (see :help multilang-menus) / :set langmenu (#1099)
• Add Apple official translations for common menu names like "Copy" or "Font" that MacVim uses (#1099)
• Translate remaining menu items.

User Interface (e.g. preference pane)

• Need translations.

Documentation (:help macvim / gui_mac.txt)

• Need translations

这是一个涵盖 Vim 消息、MacVim 特有消息、菜单、偏好设置面板以及 :help macvim / gui_mac.txt 中文档的总 issue。首先选择一个尚未勾选的区域，并检查相关的入口点，包括适用的 NSLocalizedString、:set lang 或 :set langmenu。选定的翻译范围完成实现且其 checklist 项完成后，即视为完成。