Epic: Localize MacVim and add translations

[ychin]

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

Messages

• [x] 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

• [x] Support Vim menu translations (see :help multilang-menus) / :set langmenu (#1099)
• [x] 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

[ychin]

One option is to use Crowdin to manage the localization if it's easier for contributors. We don't really have enough strings to justify such effort though.