Add sketch of I18n error translation

lib/liquid.rb

@@ -48,6 +48,7 @@ end
 require "liquid/version"
 require 'liquid/lexer'
 require 'liquid/parser'
+require 'liquid/i18n'
 require 'liquid/drop'
 require 'liquid/extensions'
 require 'liquid/errors'

lib/liquid/i18n.rb

@@ -0,0 +1,46 @@
+require 'yaml'
+require 'delegate'
+
+module Liquid
+  class I18n
+    class TranslationError < StandardError
+    end
+
+    def initialize(path)
+      @path = path
+    end
+
+    def translate(name, vars = {})
+      interpolate(deep_fetch_translation(name), vars)
+    end
+    alias_method :t, :translate
+
+    class << self
+      def translate(name, vars = {})
+        @@global.translate(name, vars)
+      end
+      alias_method :t, :translate
+
+      def global=(translator)
+        @@global = translator
+      end
+    end
+
+    private
+    def interpolate(name, vars)
+      name.gsub(/:(\w+)/) do
+        vars[$1.to_sym] or raise TranslationError, translate("errors.i18n.undefined_interpolation", :key => $1, :name => name)
+      end
+    end
+
+    def deep_fetch_translation(name)
+      name.split('.').reduce(locale) do |level, cur|
+        level[cur] or raise TranslationError, translate("errors.i18n.unknown_translation", :name => name)
+      end
+    end
+
+    def locale
+      @locale ||= YAML.load_file(@path)
+    end
+  end
+end

lib/liquid/locales/en.yml

@@ -0,0 +1,9 @@
+---
+  errors:
+    i18n:
+      unknown_translation: "Translation for :name does not exist in locale"
+      undefined_interpolation: "Undefined key :key for interpolation in translation :name"
+    template:
+      argument_hash_or_context: "Expect Hash or Liquid::Context as parameter"
+    syntax_error:
+      tag_termination: "Tag ':token' was not properly terminated with regexp: :inspection"

lib/liquid/template.rb

@@ -119,7 +119,7 @@ module Liquid
       when nil
         Context.new(assigns, instance_assigns, registers, @rethrow_errors, @resource_limits)
       else
-        raise ArgumentError, "Expect Hash or Liquid::Context as parameter"
+        raise ArgumentError, I18n.translate("errors.template.argument_hash_or_context")
       end
 
       case args.last