diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 1f85c98bec..7adcf2579e 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -258,10 +258,12 @@ jobs:
           version: 0.16.0
       - run: sudo apt-get install -y inotify-tools
 
+      - run: mkdir -p build/ # build/ is used for shared log paths, see global ENV defs
       - run: zig build $OPTS test_nlua0
       - run: zig build $OPTS nvim_bin && ./zig-out/bin/nvim --version
       - run: zig build $OPTS unittest
       - run: zig build $OPTS functionaltest
+      - run: zig build $OPTS oldtest
       # `zig build` uses a lua script for doctags in order to support cross-compiling
       # compare with the builtin generator that they match
       - run: cd runtime; ../zig-out/bin/nvim -u NONE -i NONE -e --headless -c "helptags ++t doc" -c quit
@@ -271,6 +273,8 @@ jobs:
     runs-on: macos-15
     timeout-minutes: 45
     name: build using zig build (macos 15)
+    env:
+      OPTS: # empty
     steps:
       - uses: actions/checkout@v6
         with:
@@ -280,9 +284,11 @@ jobs:
         with:
           version: 0.16.0
 
-      - run: zig build test_nlua0 -Dluajit=false
-      - run: zig build nvim_bin -Dluajit=false && ./zig-out/bin/nvim --version
-      - run: zig build functionaltest -Dluajit=false
+      - run: mkdir -p build/ # build/ is used for shared log paths, see global ENV defs
+      - run: zig build $OPTS test_nlua0
+      - run: zig build $OPTS nvim_bin && ./zig-out/bin/nvim --version
+      - run: zig build $OPTS functionaltest
+      - run: zig build $OPTS oldtest
 
   zig-build-windows:
     runs-on: windows-2025
@@ -297,6 +303,7 @@ jobs:
         with:
           version: 0.16.0
 
+      - run: mkdir -p build/ # build/ is used for shared log paths, see global ENV defs
       - run: zig build test_nlua0
       - run: zig build nvim_bin
       - run: ./zig-out/bin/nvim --version
diff --git a/build.zig.zon b/build.zig.zon
index fd5326dac6..938546f673 100644
--- a/build.zig.zon
+++ b/build.zig.zon
@@ -6,8 +6,8 @@
 
     .dependencies = .{
         .zlua = .{
-            .url = "git+https://github.com/natecraddock/ziglua#8f271c82baa5fc43aa02a72f6da020c2025d9436",
-            .hash = "zlua-0.1.0-hGRpC2aABQD4D9PBVH3wAW8k32-I4969MRQ0CpOwoley",
+            .url = "git+https://github.com/natecraddock/ziglua#1dcb9f2b7b466a1609cc11e746c2efc4f17685da",
+            .hash = "zlua-0.1.0-hGRpC22ABQA4y12Uz7QaHuCkKrpaa_66xidApjtFuKFg",
         },
         .lpeg = .{
             .url = "https://github.com/neovim/deps/raw/d495ee6f79e7962a53ad79670cb92488abe0b9b4/opt/lpeg-1.1.0.tar.gz",
diff --git a/runtime/lua/vim/treesitter.lua b/runtime/lua/vim/treesitter.lua
index 5a982deb0d..8e63750dd2 100644
--- a/runtime/lua/vim/treesitter.lua
+++ b/runtime/lua/vim/treesitter.lua
@@ -97,12 +97,12 @@ function M.get_parser(buf, lang, opts)
     if not api.nvim_buf_is_loaded(buf) then
       return nil, string.format('Buffer %s must be loaded to create parser', buf)
     end
-    local parser = vim.npcall(M._create_parser, buf, lang, opts)
-    if not parser then
-      return nil,
-        string.format('Parser could not be created for buffer %s and language "%s"', buf, lang)
+    local status, res = pcall(M._create_parser, buf, lang, opts)
+    if not status then
+      local msg = 'Parser could not be created for buffer %s and language "%s": %s'
+      return nil, string.format(msg, buf, lang, res)
     end
-    parsers[buf] = parser
+    parsers[buf] = res
   end
 
   parsers[buf]:register_cbs(opts.buf_attach_cbs)
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index bc1342d4f5..c0339e9e1d 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -212,9 +212,7 @@ void eval_clear(void)
 {
   evalvars_clear();
   free_scriptnames();  // must come after evalvars_clear().
-# ifdef HAVE_WORKING_LIBINTL
   free_locales();
-# endif
 
   // autoloaded script names
   free_autoload_scriptnames();
diff --git a/src/nvim/os/lang.c b/src/nvim/os/lang.c
index 81efaa7f1a..39a369babc 100644
--- a/src/nvim/os/lang.c
+++ b/src/nvim/os/lang.c
@@ -338,8 +338,15 @@ char *get_locales(expand_T *xp, int idx)
 
 void lang_init(void)
 {
-#if defined(__APPLE__) && !defined(ZIG_BUILD)
+#if defined(__APPLE__)
   if (!os_env_exists("LANG", true)) {
+# if defined(ZIG_BUILD)
+    // fuck it just use the objectively correct values:
+    os_setenv("LANG", "C.UTF-8", true);
+    setlocale(LC_ALL, "C.UTF-8");
+    // Make sure strtod() uses a decimal point, not a comma.
+    setlocale(LC_NUMERIC, "C");
+# else
     char buf[50] = { 0 };
 
     // $LANG is not set, either because it was unset or Nvim was started
@@ -358,6 +365,7 @@ void lang_init(void)
     } else {
       ELOG("$LANG is empty and the macOS primary language cannot be inferred.");
     }
+# endif
   }
 #endif
 }
diff --git a/src/nvim/spellfile.c b/src/nvim/spellfile.c
index aa7565fd75..c3774cf293 100644
--- a/src/nvim/spellfile.c
+++ b/src/nvim/spellfile.c
@@ -1789,7 +1789,7 @@ static idx_T read_tree_node(FILE *fd, uint8_t *byts, idx_T *idxs, int maxidx, id
             c = (getc(fd) << 16) + c;                   // <region>
           }
           if (c & WF_AFX) {
-            c = (getc(fd) << 24) + c;                   // <affixID>
+            c = (int)((unsigned)getc(fd) << 24) + c;  // <affixID>
           }
         }
 
diff --git a/src/nvim/usercmd.c b/src/nvim/usercmd.c
index 4c7305e2a9..0a9a6ce9c0 100644
--- a/src/nvim/usercmd.c
+++ b/src/nvim/usercmd.c
@@ -77,9 +77,7 @@ static const char *command_complete[] = {
   [EXPAND_HIGHLIGHT] = "highlight",
   [EXPAND_HISTORY] = "history",
   [EXPAND_KEYMAP] = "keymap",
-#ifdef HAVE_WORKING_LIBINTL
   [EXPAND_LOCALES] = "locale",
-#endif
   [EXPAND_LUA] = "lua",
   [EXPAND_MAPCLEAR] = "mapclear",
   [EXPAND_MAPPINGS] = "mapping",
diff --git a/test/functional/treesitter/language_spec.lua b/test/functional/treesitter/language_spec.lua
index f01530a24b..96305a611a 100644
--- a/test/functional/treesitter/language_spec.lua
+++ b/test/functional/treesitter/language_spec.lua
@@ -19,7 +19,10 @@ describe('treesitter language API', function()
       return vim.treesitter.get_parser(0, 'borklang')
     end)
     eq(NIL, parser)
-    eq('Parser could not be created for buffer 1 and language "borklang"', error)
+    matches(
+      '^Parser could not be created for buffer 1 and language "borklang": .+: No parser for language "borklang"$',
+      error
+    )
 
     -- actual message depends on platform
     matches(
@@ -110,7 +113,10 @@ describe('treesitter language API', function()
         return vim.treesitter.get_parser(0, 'borklang')
       end)
       eq(NIL, parser)
-      eq('Parser could not be created for buffer 1 and language "borklang"', error)
+      matches(
+        '^Parser could not be created for buffer 1 and language "borklang": .+: No parser for language "borklang"$',
+        error
+      )
     end
   )
 
diff --git a/test/old/runner.lua b/test/old/runner.lua
new file mode 100644
index 0000000000..e8558cc39a
--- /dev/null
+++ b/test/old/runner.lua
@@ -0,0 +1,160 @@
+-- execute old style tests
+
+-- when compared to the testdir/Makefile based approach, this replaces
+-- all wrapper process logic:
+-- Makefile
+-- runnvim.sh and its library test.sh
+-- runnvim.vim (wrapper which runs a nested vim inside a terminal)
+
+-- it DOES NOT not replace the logic running inside the nvim-under-test
+-- runtest.vim and its libraries setup.vim, shared.vim, check.vim
+
+local nvim_bin = vim.v.progpath -- must be absolute
+
+vim.api.nvim_set_current_dir('test/old/testdir/')
+
+local function readable(fn)
+  return vim.fn.filereadable(fn) ~= 0
+end
+
+local errlog = 'test.log'
+local messages = 'messages'
+local starttime = 'starttime'
+local gen_opt_log = 'gen_opt_test.log'
+
+function oksystem(args)
+  local res = vim.system(args, {}):wait()
+  if res.code ~= 0 then
+    error(vim.inspect(args) .. ' failed:\n' .. res.stdout .. res.stderr)
+  end
+  return res
+end
+
+function clean_output()
+  for _, f in ipairs { errlog, messages, starttime, gen_opt_log, 'opt_test.vim' } do
+    vim.uv.fs_unlink(f)
+  end
+  -- check status, some tests mess with perms so this could fail:
+  oksystem { 'bash', '-c', 'rm -rf X* viminfo test.out test.ok' }
+end
+
+function if_nonempty(rawtext, name)
+  text = vim.trim(rawtext)
+  if #text > 0 then
+    if name then
+      print('== ' .. name)
+    end
+    print(text)
+    print(name and '== END ' .. name or '\n')
+  end
+end
+
+function if_exists(fn, name)
+  local fil = io.open(fn)
+  if fil then
+    if_nonempty(fil:read '*a', name)
+    fil:close()
+    return true
+  end
+end
+
+local verbose = vim.env.VERBOSE
+
+function run_nvim(...)
+  local cmd = vim.system(
+    { nvim_bin, '-u', 'NONE', '-i', 'NONE', '--headless', ... },
+    { env = { VIMRUNTIME = '../../../runtime' } }
+  )
+  return cmd:wait()
+end
+
+function run_test(testfile)
+  if not vim.endswith(testfile, '.vim') then
+    error 'not an oldtest'
+  end
+
+  clean_output()
+
+  if testfile == 'test_options_all.vim' then
+    local res = run_nvim(
+      '-S',
+      'gen_opt_test.vim',
+      '../../../src/nvim/options.lua',
+      '../../../runtime/doc/options.txt'
+    )
+
+    if if_exists(gen_opt_log, 'GEN opt_test.vim') or res.code > 0 then
+      return false
+    end
+  end
+
+  local resfile = string.sub(testfile, 1, -4) .. 'res'
+  vim.uv.fs_unlink(resfile)
+  assert(not readable(resfile))
+
+  local opts = 'set shortmess-=F backupdir=. undodir=. viewdir=.'
+  local res = run_nvim('--cmd', opts, '-S', 'runtest.vim', testfile)
+  local passed = (readable(resfile) and res.code == 0)
+
+  if_exists(messages)
+
+  if verbose or not passed then
+    if_nonempty(res.stdout, 'STDOUT')
+    if_nonempty(res.stderr, 'STDERR')
+    -- if_exists(starttime, "starttime")
+    if_exists(errlog, 'test.log')
+  end
+  return passed
+end
+
+local function all_tests()
+  local nested_tests = {}
+  local alot = io.open('test_alot.vim')
+  local sourcepat = '^source (test_.*%.vim)$'
+  for line in alot:lines() do
+    local m = string.match(line, sourcepat)
+    if m then
+      nested_tests[m] = true
+    end
+  end
+  alot:close()
+
+  local files = {}
+  for name, type in vim.fs.dir('.') do
+    match = string.match(name, '^test_.*%.vim$')
+    if match then
+      if nested_tests[match] then
+        nested_tests[match] = nil
+      elseif match == 'test_largefile.vim' then
+        -- ignored: uses too much resources to run on CI
+      else
+        table.insert(files, match)
+      end
+    end
+  end
+  return files
+end
+
+local files
+if #arg > 0 then
+  files = {}
+  for _, a in ipairs(arg) do
+    table.insert(files, string.match(a, '[^/]*$'))
+  end
+else
+  files = all_tests()
+end
+
+local count = #files
+local failures = {}
+for i, fil in ipairs(files) do
+  print('run: ', fil)
+  if not run_test(fil) then
+    table.insert(failures, fil)
+  end
+end
+
+if #failures > 0 then
+  print('failed:', vim.inspect(failures))
+  vim.cmd 'cquit'
+end
diff --git a/test/run_tests.zig b/test/run_tests.zig
index 39238b4473..1c864e8b79 100644
--- a/test/run_tests.zig
+++ b/test/run_tests.zig
@@ -50,6 +50,19 @@ pub fn test_steps(b: *std.Build, nvim_bin: *std.Build.Step.Compile, depend_on: *
     const functionaltest_step = b.step("functionaltest", "run functional tests");
     functionaltest_step.dependOn(&functional_tests.step);
 
+    const old_tests = b.addRunArtifact(nvim_bin);
+    old_tests.addArg("-l");
+    old_tests.addFileArg(b.path("./test/old/runner.lua"));
+    if (b.args) |args| {
+        old_tests.addArgs(args); // accept TEST_FILE as a positional argument
+    }
+    const env = old_tests.getEnvMap();
+    try env.put("BUILD_DIR", b.install_path);
+
+    const oldtest_step = b.step("oldtest", "run old tests");
+    oldtest_step.dependOn(&old_tests.step);
+    oldtest_step.dependOn(depend_on);
+
     if (unit_paths) |paths| {
         const unit_tests = try testStep(b, "unit", nvim_bin, config_dir, paths);
         unit_tests.step.dependOn(depend_on);