Hello,
The codes of po-mode.el refers obsolete variable font-lock-reference-face, currently.
This works fine in the current release version of Emacs(Emacs 28.x), because this version refers font-lock-reference-face by using aliasing-mechanism to refer obsolete variables.
Thanks.
---
diff --git a/gettext-tools/emacs/po-mode.el b/gettext-tools/emacs/po-mode.el
index e04428772..162e18f63 100644
--- a/gettext-tools/emacs/po-mode.el
+++ b/gettext-tools/emacs/po-mode.el
@@ -909,7 +909,7 @@ M-S Ignore path M-A Ignore PO file *M-L Ignore lexicon
(defconst po-font-lock-keywords
'(
("^# .*\\|^#[:,]?" . font-lock-comment-face)
- ("^#:\\(.*\\)" 1 font-lock-reference-face)
+ ("^#:\\(.*\\)" 1 font-lock-constant-face)
("^#,\\(.*\\)" 1 font-lock-function-name-face)
("^\\(\\(msg\\(ctxt\\|id\\(_plural\\)?\\|str\\(\\[[0-9]\\]\\)?\\)\\) \\)?\"\\|\"$"
. font-lock-keyword-face)