diff --git a/gtfs/spec/en/reference.md b/gtfs/spec/en/reference.md
index cb905621..c0f5b008 100644
--- a/gtfs/spec/en/reference.md
+++ b/gtfs/spec/en/reference.md
@@ -83,7 +83,7 @@ Presence conditions applicable to fields and files:
 - **Email** - An email address. <br> *Example: `example@example.com`*
 - **Enum** - An option from a set of predefined constants defined in the "Description" column. <br> *Example: The `route_type` field contains a `0` for tram, a `1` for subway...*
 - **ID** - An ID field value is an internal ID, not intended to be shown to riders, and is a sequence of any UTF-8 characters. Using only printable ASCII characters is recommended. An ID is labeled "unique ID" when it must be unique within a file. IDs defined in one .txt file are often referenced in another .txt file. IDs that reference an ID in another table are labeled "foreign ID".<br> *Example: The `stop_id` field in [stops.txt](#stopstxt) is a "unique ID". The `parent_station` field in [stops.txt](#stopstxt) is a "foreign ID referencing `stops.stop_id`".*
-- **Language code** - An IETF BCP 47 language code. For an introduction to IETF BCP 47, refer to [http://www.rfc-editor.org/rfc/bcp/bcp47.txt](http://www.rfc-editor.org/rfc/bcp/bcp47.txt) and [http://www.w3.org/International/articles/language-tags/](http://www.w3.org/International/articles/language-tags/). <br> *Example: `en` for English, `en-US` for American English or `de` for German.*
+- **Language code** - An IETF BCP 47 language code. For an introduction to IETF BCP 47, refer to [https://www.rfc-editor.org/info/rfc5646/](https://www.rfc-editor.org/info/rfc5646/) and [http://www.w3.org/International/articles/language-tags/](http://www.w3.org/International/articles/language-tags/). <br> *Example: `en` for English, `en-US` for American English or `de` for German.*
 - **Latitude** - WGS84 latitude in decimal degrees. The value must be greater than or equal to -90.0 and less than or equal to 90.0. *<br> Example: `41.890169` for the Colosseum in Rome.*
 - **Longitude** - WGS84 longitude in decimal degrees. The value must be greater than or equal to -180.0 and less than or equal to 180.0. <br> *Example: `12.492269` for the Colosseum in Rome.*
 - **Float** - A floating point number.