checkhealth: validate locale (#9548)

Test case:

    LANG= LC_ALL= LC_CTYPE= tmux
    nvim -u NORC +'checkhealth nvim'

runtime/autoload/health.vim

@@ -30,7 +30,7 @@ function! health#check(plugin_names) abort
         \ : s:to_fn_names(a:plugin_names)
 
   tabnew
-  setlocal wrap breakindent
+  setlocal wrap breakindent linebreak
   setlocal filetype=markdown
   setlocal conceallevel=2 concealcursor=nc
   setlocal keywordprg=:help

runtime/autoload/health/nvim.vim

@@ -25,6 +25,15 @@ function! s:check_config() abort
           \   'https://github.com/neovim/neovim/wiki/Following-HEAD#20170402' ])
   endif
 
+  if v:ctype ==# 'C'
+    let ok = v:false
+    call health#report_error('Locale does not support UTF-8. Unicode characters may not display correctly.'
+          \                  .printf("\n$LANG=%s $LC_ALL=%s $LC_CTYPE=%s", $LANG, $LC_ALL, $LC_CTYPE),
+          \ [ 'If using tmux, try the -u option.',
+          \   'Ensure that your terminal/shell/tmux/etc inherits the environment, or set $LANG explicitly.' ,
+          \   'Configure your system locale.' ])
+  endif
+
   if &paste
     let ok = v:false
     call health#report_error("'paste' is enabled. This option is only for pasting text.\nIt should not be set in your config.",