Implemented #24 by adding option --only-untranslated to all formatters (that adopt the modular style).

lib/twine/cli.rb

@@ -61,6 +61,9 @@ module Twine
         opts.on('-x', '--exclude-untranslated', 'This flag will cause any string files that are generated to not include strings that have not yet been translated for the current language.') do |s|
           @options[:exclude_untranslated] = true
         end
+        opts.on('-u', '--only-untranslated', 'This flag will cause any generated files to include only the strings that have not been translated.') do |c|
+          @options[:only_untranslated] = true
+        end
         opts.on('-o', '--output-file OUTPUT_FILE', 'Write the new strings database to this file instead of replacing the original file. This flag is only useful when running the consume-string-file or consume-loc-drop commands.') do |o|
           @options[:output_path] = o
         end

lib/twine/formatters/abstract.rb

@@ -157,9 +157,13 @@ module Twine
 
       def format_row(row, lang)
         value = row.translated_string_for_lang(lang)
-        if value.nil? and not @options[:exclude_untranslated]
+
+        return if value && @options[:only_untranslated]
+
+        if value.nil? && (!@options[:exclude_untranslated] || @options[:only_untranslated])
           value = row.translated_string_for_lang(fallback_languages(lang))
         end
+
         return nil unless value
 
         result = ""

test/fixtures/test-output-14.txt

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Android Strings File -->
+<!-- Generated by Twine <%= Twine::VERSION %> -->
+<!-- Language: fr -->
+<resources>
+	<!-- SECTION: My Strings -->
+	<string name="key3">key3-english</string>
+	<string name="key4">key4-english</string>
+</resources>

test/twine_test.rb

@@ -139,4 +139,12 @@ class TwineTest < Test::Unit::TestCase
       assert_equal(File.read('test/fixtures/test-json-line-breaks/generated.json'), File.read(output_path))
     end
   end
+
+  def test_generate_string_file_14_only_untranslated
+    Dir.mktmpdir do |dir|
+      output_path = File.join(dir, 'only_untranslated.xml')
+      Twine::Runner.run(%W(generate-string-file test/fixtures/strings-1.txt #{output_path} --only-untranslated -l fr))
+      assert_equal(ERB.new(File.read('test/fixtures/test-output-14.txt')).result, File.read(output_path))
+    end
+  end
 end