diff --git a/README.md b/README.md
index 04851682e..2ecf9b5e9 100644
--- a/README.md
+++ b/README.md
@@ -36,11 +36,26 @@ These configuration preferences indicate
 - what i18n languages you wish to support
 - what is your default "fallback" language for your content
 - what root level files/folders are excluded from localization, based on if their paths start with any of the excluded regexp substrings. (this is different from the jekyll `exclude: [ .gitignore ]` ; you should `exclude` files and directories in your repo you dont want in your built site at all, and `exclude_from_localization` files and directories you want to see in your built site, but not in your sublanguage sites.)
-- whether to run language processing in parallel or serial. Set to `false` if building on Windows hosts, or if Polyglot collides with other Jekyll plugins.
+- whether to run language processing in parallel or serial. Set to `false` if building on Windows hosts, or if Polyglot collides with other Jekyll plugins. If a plugin breaks only when you turn this on, try [`serial_default_lang`](#parallel-safe-plugins-serial_default_lang) before giving up on parallel builds.
 - your jekyll website production url. Make sure this value is set; Polyglot requires this to relative site urls correctly, and to make functioning language switchers.
 
 The optional `lang_from_path: true` option enables getting the page language from a filepath segment seperated by `/` or `.`, e.g `de/first-one.md`, or `_posts/zh_HK/use-second-segment.md` , if the lang frontmatter isn't defined.
 
+#### Parallel-safe plugins (`serial_default_lang`)
+
+```yaml
+parallel_localization: true
+serial_default_lang: true
+```
+
+Default: `false`. Has no effect when `parallel_localization` is `false`.
+
+With `parallel_localization` on, polyglot forks one process per language and runs them at once. Some plugins aren't safe to run that way — typically ones that do expensive setup once per build and share state (a cache, a manifest, a generated directory) across the whole site. When every fork tries to do that setup at the same time, they race and the build fails. [`jekyll-assets`](https://github.com/envygeeks/jekyll-assets) is the common example.
+
+`serial_default_lang: true` makes polyglot build the default language first, on its own, before forking the rest. The expensive one-time setup happens once in that first pass, and the language forks inherit the finished state instead of each redoing it. Enable this if a plugin that works fine with `parallel_localization: false` breaks when you turn it on.
+
+The trade-off is small: the default language no longer runs alongside the others, so you get one fewer concurrent fork.
+
 #### Netlify _redirects localization
 If you are deploying to Netlify and use a `_redirects` file, you can enable automatic localization of redirects:
 ```yaml
diff --git a/lib/jekyll/polyglot/patches/jekyll/site.rb b/lib/jekyll/polyglot/patches/jekyll/site.rb
index 06aee3a41..c242bf4e0 100644
--- a/lib/jekyll/polyglot/patches/jekyll/site.rb
+++ b/lib/jekyll/polyglot/patches/jekyll/site.rb
@@ -3,13 +3,20 @@
 include Process
 module Jekyll
   class Site
-    attr_reader :default_lang, :languages, :exclude_from_localization, :lang_vars, :lang_from_path, :fallback_canonical_to_default_lang
+    attr_reader :default_lang, :languages, :exclude_from_localization, :lang_vars, :lang_from_path, :fallback_canonical_to_default_lang, :serial_default_lang
     attr_accessor :file_langs, :active_lang
 
     def prepare
       @file_langs = {}
       fetch_languages
       @parallel_localization = config.fetch('parallel_localization', true)
+      # When true (and parallel_localization is also true), the default
+      # language is processed synchronously in the parent before any forks
+      # are spawned for the other languages. This makes parallel builds
+      # safe for plugins that do expensive one-time setup and share state
+      # across the site (e.g. jekyll-assets), which otherwise race when
+      # every fork runs that setup at once. See README for details.
+      @serial_default_lang = config.fetch('serial_default_lang', false)
       @lang_from_path = config.fetch('lang_from_path', false)
       @fallback_canonical_to_default_lang = config.fetch('fallback_canonical_to_default_lang', false)
       @exclude_from_localization = config.fetch('exclude_from_localization', []).map do |e|
@@ -34,14 +41,23 @@ def process
       prepare
       all_langs = ([@default_lang] + @languages).uniq
       if @parallel_localization
+        if @serial_default_lang
+          # Run the default language in the parent first to prime shared
+          # state (e.g. jekyll-assets' Sprockets cache) before forking
+          # for the remaining languages.
+          process_language @default_lang
+          langs_to_fork = @languages - [@default_lang]
+        else
+          langs_to_fork = all_langs
+        end
         nproc = Etc.nprocessors
         pids = {}
         begin
-          all_langs.each do |lang|
+          langs_to_fork.each do |lang|
             pids[lang] = fork do
               process_language lang
             end
-            while pids.length >= (lang == all_langs[-1] ? 1 : nproc)
+            while pids.length >= (lang == langs_to_fork[-1] ? 1 : nproc)
               sleep 0.1
               pids.map do |pid_lang, pid|
                 next unless waitpid pid, Process::WNOHANG
@@ -52,7 +68,7 @@ def process
             end
           end
         rescue Interrupt
-          all_langs.each do |lang|
+          langs_to_fork.each do |lang|
             next unless pids.key? lang
 
             puts "Killing #{pids[lang]} : #{lang}"
diff --git a/spec/jekyll/polyglot/patches/jekyll/site_spec.rb b/spec/jekyll/polyglot/patches/jekyll/site_spec.rb
index fcddd7a7c..134e919df 100644
--- a/spec/jekyll/polyglot/patches/jekyll/site_spec.rb
+++ b/spec/jekyll/polyglot/patches/jekyll/site_spec.rb
@@ -413,6 +413,39 @@
       expect(forks).to eq((@langs + [@default_lang]).uniq.length)
     end
 
+    it 'runs the default language in the parent when serial_default_lang is set' do
+      site_with_serial = Site.new(
+        Jekyll.configuration(
+          'languages'                 => @langs,
+          'default_lang'              => @default_lang,
+          'exclude_from_localization' => @exclude_from_localization,
+          'serial_default_lang'       => true,
+          'source'                    => File.expand_path('fixtures', __dir__),
+          'url'                       => 'https://test.github.io'
+        )
+      )
+
+      parent_lang_calls = []
+      allow(Etc).to receive(:nprocessors).and_return(99)
+      # Real but immediate forks so polyglot's waitpid loop terminates.
+      allow(site_with_serial).to receive(:fork) { fork { exit 0 } }
+      # Track parent-process calls to process_language. Calls inside fork
+      # blocks happen in child processes (with copied state) and don't
+      # show up in this list.
+      allow(site_with_serial).to receive(:process_language) do |lang|
+        parent_lang_calls << lang
+      end
+
+      site_with_serial.process
+
+      expect(site_with_serial.serial_default_lang).to be true
+      # The default language is the only one processed in the parent —
+      # all others are dispatched to fork blocks. This is the whole point
+      # of the option: prime shared on-disk state once in the parent,
+      # then let forks inherit it via fork(2) copy-on-write.
+      expect(parent_lang_calls).to eq([@default_lang])
+    end
+
     describe 'assignPageRedirects' do
       before do
         @collection = Jekyll::Collection.new(@site, 'test')