diff --git a/lib/converter.rb b/lib/converter.rb
index d9e2a1e..9511e1a 100644
--- a/lib/converter.rb
+++ b/lib/converter.rb
@@ -19,11 +19,12 @@ def initialize(storage_dir: C_.storage_directory, logger: Logger.new(C_.conversi
     # @param version [String] the file version
     # @param filename [String] the name of the file to convert
     # @param action [String] the conversion to perform
+    # @param language [String] the file's language. Forced to `en` if `nil`
     # @return the converted file name
-    def convert(doc_key, version, filename, action)
+    def convert(doc_key, version, filename, action, language = nil)
+      language ||= 'en'
       doc = Document.load @storage_dir, doc_key
       ignore, orig_content = doc.get_file(version, filename)
-      language = 'en' # TODO: add to spec and upload
       new_content = convert_file action, orig_content, language
       # TODO: handling for variant formats with the same extension
       #       probably by adding format info before suffix
@@ -37,9 +38,10 @@ def convert(doc_key, version, filename, action)
     # Converts the supplied content. Nothing gets saved.
     # @param action [String] the conversion to perform
     # @param orig_content [String] the body of the file to convert
-    # @param language [String] the file's language
+    # @param language [String] the file's language. Forced to `en` if `nil`
     # @return [String] the converted file body
-    def convert_file(action, orig_content, language = 'en')
+    def convert_file(action, orig_content, language = nil)
+      language ||= 'en'
       Heathen::Converter.new(logger: @logger).convert(action, orig_content, language)
     rescue Heathen::TaskNotFound => e
       raise InvalidAction.new(e.message)
diff --git a/lib/heathen/converter.rb b/lib/heathen/converter.rb
index a57832f..667f9d5 100644
--- a/lib/heathen/converter.rb
+++ b/lib/heathen/converter.rb
@@ -13,9 +13,10 @@ def initialize(logger: Logger.new(nil))
     # Converts the given document according to the action requested.
     # @param action [String] the conversion action to perform
     # @param content [String] the document body to be converted
-    # @param language [String] the document language (defaults to 'en')
+    # @param language [String] the file's language. Forced to `en` if `nil`
     # @return [String] the converted document body
-    def convert(action, content, language = 'en')
+    def convert(action, content, language = nil)
+      language ||= 'en'
       job = Job.new action, content, language
       processor = Heathen::Processor.new job: job, logger: @logger
       begin
diff --git a/lib/heathen/job.rb b/lib/heathen/job.rb
index 054b515..8460156 100644
--- a/lib/heathen/job.rb
+++ b/lib/heathen/job.rb
@@ -25,11 +25,11 @@ class Job
     # Constructs a new job
     # @param action [String] the action to be performed
     # @param content [String] the file body
-    # @param language [String] the file's language
+    # @param language [String] the file's language. Forced to `en` if `nil`
     # @param sandbox_dir [String] sandbox directory for temporary files
-    def initialize(action, content, language = 'en', sandbox_dir = nil)
+    def initialize(action, content, language = nil, sandbox_dir = nil)
       @action = action
-      @language = language
+      @language = language || 'en'
       @original_content = content
       @original_mime_type = content.mime_type
       self.content = @original_content
diff --git a/lib/legacy_converter.rb b/lib/legacy_converter.rb
index 6fcde99..8315985 100644
--- a/lib/legacy_converter.rb
+++ b/lib/legacy_converter.rb
@@ -27,9 +27,10 @@ def initialize(storage_dir = C_.storage_directory)
     # Converts the given file and stores it in the legacy directory
     # @param action [String] the conversion to perform
     # @param orig_content [String] the body of the file to convert
-    # @param language [String] the file's language
+    # @param language [String] the file's language. Forced to `en` if `nil`
     # @return [String] the path to the converted file
-    def convert_file(action, orig_content, language = 'en')
+    def convert_file(action, orig_content, language = nil)
+      language ||= 'en'
       content = Heathen::Converter.new.convert(action, orig_content, language)
       filename = Digest::SHA2.hexdigest content
       store_file filename, content