diff --git a/.readthedocs.yml b/.readthedocs.yml
index 915a7cd132..14b4364905 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -15,5 +15,5 @@ build:
       sphinx-build ./sphinx/doc/ ./_readthedocs/html 
       -b html  
       -D locale_dirs=$(realpath locale)
-      -D language=$READTHEDOCS_LANGUAGE
+      -D language=$(python convert_language_tag.py)
       -D gettext_compact=0
diff --git a/convert_language_tag.py b/convert_language_tag.py
new file mode 100755
index 0000000000..9324ea7c9a
--- /dev/null
+++ b/convert_language_tag.py
@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+"""
+Convert a READTHEDOCS_LANGUAGE (ll-cc) to Sphinx-compatible format (ll_CC),
+and print the result.
+
+This script reads Read the Docs's READTHEDOCS_LANGUAGE environment variable,
+and convert ll-cc into ll_CC.
+
+The output is the same as input if the input language has only language
+without country part ('ll') or is already Gettext's ISO 639 (ll_CC).
+
+Exit with error in case READTHEDOCS_LANGUAGE is not set or th input is not
+one of the valid language formats (ll, ll-cc or ll_CC)
+"""
+
+import os
+import re
+import sys
+
+IETF_LANG_TAG_PATTERN = r"^[a-z]{2}-[a-z]{2}$"
+A_LANG_TAG_PATTERN = r"^[a-z]{2}(-[a-z]{2}|_[A-Z]{2})?$"
+
+
+def match_language_tag(input_str: str, pattern: str) -> bool:
+    """Match the *input_str* string with the language tag *pattern*."""
+    return bool(re.match(pattern, input_str))
+
+
+def convert_to_iso639_gettext(input_str: str) -> str:
+    """Convert *input_str* from ll-cc to ll_CC language format."""
+    first_part, _, last_part = input_str.partition("-")
+    return f"{first_part}_{last_part.upper()}"
+
+
+def main():
+    rtd_language = os.getenv("READTHEDOCS_LANGUAGE")
+    if not rtd_language:
+        sys.exit(
+            "Error: READTHEDOCS_LANGUAGE not set, "
+            + "is this being run in readthedocs build env?"
+        )
+
+    if not match_language_tag(rtd_language, A_LANG_TAG_PATTERN):
+        sys.exit(
+            "Error: Expected a language code like pt, pt_BR or pt-br. "
+            f"Invalid language code given: {rtd_language}"
+        )
+
+    output_language = (
+        convert_to_iso639_gettext(rtd_language)
+        if match_language_tag(rtd_language, IETF_LANG_TAG_PATTERN)
+        else rtd_language
+    )
+    print(output_language)
+
+
+if __name__ == "__main__":
+    main()