Update CONTRIBUTING.md

Added "Sort channels by language" section

.github/CONTRIBUTING.md

@@ -4,6 +4,7 @@ If you want to help the project you can do this in several ways. Here are some o
 
 - [Add channel](#add-channel)
 - [Sort channels by category](#sort-channels-by-category)
+- [Sort channels by language](#sort-channels-by-language)
 - [Sort channels by country](#sort-channels-by-country)
 - [Remove broken broadcasts](#remove-broken-broadcasts)
 - [Add EPG source](#add-epg-source)
@@ -68,6 +69,28 @@ For convenience, `https://iptv-org.github.io/iptv/categories/other.m3u` contains
 
 A complete list of supported categories can be found [here](https://github.com/iptv-org/iptv#playlists-by-category).
 
+## Sort channels by language
+
+To sort the channel by language, you only need to specify the appropriate language in `tvg-language` attribute in the channel description. For example:
+
+```xml
+#EXTINF:-1 tvg-language="Arabic",Abu Dhabi Drama
+http://example.com/ad-drama.m3u8
+```
+
+And at the next update the channel will automatically get to the necessary playlist, in this case it is `languages/ara.m3u`.
+
+Importantly, the name of the language must comply with [ISO 639-3](https://iso639-3.sil.org/code_tables/639/data?title=&field_iso639_cd_st_mmbrshp_639_1_tid=94671&name_3=&field_iso639_element_scope_tid=All&field_iso639_language_type_tid=51&items_per_page=500) standart otherwise it will simply be ignored during the playlist update.
+
+If a channel is broadcast in several languages at once, you can specify them all through a semicolon, like this:
+
+```xml
+#EXTINF:-1 tvg-language="English;Chinese",CCTV
+http://example.com/cctv.m3u8
+```
+
+In case you do not know exactly which language the given channel is broadcast in, you can leave the field `tvg-language` empty. In this case, the channel will be automatically saved in `languages/undefined.m3u` file and someone from the community will be able to specify the correct language later.
+
 ## Sort channels by country
 
 You can help sorting channels by country by moving the link to the channel with the entire description from one playlist in the `channels/` folder to another. Be careful, any changes outside the `channels/` folder will not be accepted, since the rest of the playlists are generated automatically.