add lang flag and update documentation

doc/comitium.1.scd

@@ -60,6 +60,13 @@ command-specific flags that are not universally recognized.
 	Set the data directory where comitium will store its files. See *FILES AND
 	DIRECTORIES* for more information.
 
+*-l* _LANG_, *--lang* _LANG_
+	The language that comitium will use when outputting its files.  Can be a
+	comma-separated list of language tags, comitium will use the first avalable.
+	Overrides the $*LANG* environment variable.  See
+	<https://git.nytpu.com/comitium/about/doc/translating.md> for available
+	options.  Defaults to en-US.
+
 ## REFRESH FLAGS
 
 *-n* _NUM_, *--num-workers* _NUM_
@@ -91,6 +98,9 @@ command-specific flags that are not universally recognized.
 *COMITIUM_DATA*
 	Set the data directory. Equivalent to the *--data* flag.
 
+*LANG*
+	Set the language comitium will output.
+
 # FILES AND DIRECTORIES
 
 ## DATA DIRECTORY

flags.go

@@ -11,6 +11,7 @@ var (
 	globalFs = flag.NewFlagSet("global", flag.ContinueOnError)
 
 	dataFlag = globalFs.StringP("data", "d", "", "The data directory where comitium will store its files")
+	langFlag = globalFs.StringP("lang", "l", "en-US", "The language that comitium will use when outputting its files")
 )
 
 // command holds fields used for printing information about a command.