Fix MacVim's locale not having encoding when launched from Dock

When launching MacVim not from terminal, macOS doesn't set the $LANG
environment variable, and the locale API doesn't have a way to return
the encoding (probably because everything is UTF-8). As such Vim would
set the locale to something like "en_US", which breaks certain tools
that expects an encoding part set.

Fix this by simply appending ".UTF-8" to the constructed locale (e.g.
"en_US.UTF-8") if $LANG doesn't exist. This makes sense as we default to
UTF-8 in MacVim and macOS is basically UTF-8 native anyway.

Also, add a test that will make sure this is the case.

Fix #1033

src/os_mac_conv.c

@@ -576,6 +576,16 @@ mac_lang_init(void)
 		    kLocaleRegionMask | kLocaleRegionVariantMask,
 		    sizeof buf, buf) == noErr && *buf)
 	{
+#   ifdef FEAT_GUI_MACVIM
+	    // This usually happens when the user directly launches from the Dock
+	    // instead of terminal. macOS doesn't really provide the encoding part
+	    // in the locale API, since it assumes everything is UTF-8 anyway. We
+	    // should manually append a UTF-8 encoding component to the locale
+	    // string. This helps tools that wants to parse the encoding compoennt
+	    // of the locale.
+	    strlcat(buf, ".UTF-8", sizeof(buf)/sizeof(char));
+#   endif
+        
 	    vim_setenv((char_u *)"LANG", (char_u *)buf);
 #   ifdef HAVE_LOCALE_H
 	    setlocale(LC_ALL, "");

src/testdir/test_macvim.vim

@@ -1,6 +1,7 @@
 " Test for MacVim behaviors and regressions
 
 source check.vim
+source term_util.vim
 CheckFeature gui_macvim
 
 " Tests for basic existence of commands and options to make sure no
@@ -54,3 +55,43 @@ func Test_macvim_mappings()
     call feedkeys("\<ForceClick>", "xt")
     call assert_equal(5, g:marker_value)
 endfunc
+
+" Test that we correctly set the locale to have .UTF-8 if launching from the
+" Dock or the $LANG env variable is not set.
+func Test_macvim_default_locale_utf8()
+    CheckRunVimInTerminal
+    if !has('terminal') || !executable('/bin/sh')
+        return
+    endif
+    let buf = term_start('/bin/sh')
+
+    " Wait for shell prompt.
+    call WaitForAssert({-> assert_match('[$#] $', term_getline(buf, '.'))})
+
+    " Unset the $LANG environmental variable. This causes Vim to try to set a
+    " default one in macOS.
+    call term_sendkeys(buf, "unset LANG\<CR>")
+
+    " Run Vim and ask it to output the $LANG variable. It should be
+    " automatically created since it doesn't exist.
+    call term_sendkeys(buf, v:progpath
+        \              . " --clean -X"
+        \              . " -c 'echo $LANG'\<CR>")
+
+    " Wait for Vim to come up and show something in the status line.
+    let term_rows = term_getsize(buf)[0]
+    call term_wait(buf)
+    call WaitFor({-> len(term_getline(buf, term_rows)) > 0})
+
+    " Check that the locale actually has .UTF-8 in it. We can't check for
+    " "en_US.UTF-8" because we shouldn't assume what locale the tester is
+    " using.
+    call assert_match('^[a-zA-Z-_]\+\.UTF-8\>', term_getline(buf, term_rows), "Default locale doesn't have UTF-8 encoding set")
+
+    " Cleanly exist from Vim/terminal and clean up.
+    call term_sendkeys(buf, ":qall!\<CR>")
+    call term_wait(buf)
+    call WaitForAssert({-> assert_match('[$#] $', term_getline(buf, '.'))})
+    call StopShellInTerminal(buf)
+    exe buf . 'bwipe!'
+endfunc