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

This is an umbrella issue covering Vim messages, MacVim-specific messages, menus, the preference pane, and documentation in :help macvim / gui_mac.txt. Start by selecting one unchecked area and reviewing the related entry point, including NSLocalizedString, :set lang, or :set langmenu as applicable. Done means that the selected translation scope is implemented and its checklist item is complete.