Warn with C.UTF-8 too

See these glibc changes:

********************************************************************
commit 2897b231a6b71ee17d47d3d63f1112b2641a476c
Author: Bruno Haible <bruno@clisp.org>
Date:   Mon Sep 4 15:31:36 2023 +0200

    intl: Treat C.UTF-8 locale like C locale (BZ# 16621)

    The wiki page https://sourceware.org/glibc/wiki/Proposals/C.UTF-8
    says that "Setting LC_ALL=C.UTF-8 will ignore LANGUAGE just like it
    does with LC_ALL=C." This patch implements it.

    * intl/dcigettext.c (guess_category_value): Treat C.<encoding> locale
    like the C locale.

    Reviewed-by: Florian Weimer <fweimer@redhat.com>
*********************************************************************

via

*********************************************************************
  * debian/patches/any/git-c-utf-8-language.diff: backport support from
    upstream to treat C.<encoding> locale like C locale.  Closes: #874160.
*********************************************************************

R/language.R

@@ -36,8 +36,11 @@ local_language <- function(lang, .local_envir = parent.frame()) {
   # > (or LC_ALL) to a value other than ‘C’, before you can use a language
   # > priority list through the LANGUAGE variable.
   # --- https://www.gnu.org/software/gettext/manual/html_node/The-LANGUAGE-variable.html
-  if (identical(Sys.getenv("LANG"), "C")) {
-    warning("Changing language has no effect when envvar LANG='C'")
+  if (Sys.getenv("LANG") %in% c("C", "C.UTF-8")) {
+    warning(sprintf(
+      "Changing language has no effect when envvar LANG='%s'",
+      Sys.getenv("LANG")
+    ))
   }
 
   local_envvar(LANGUAGE = lang, .local_envir = .local_envir)