Merge bitcoin/bitcoin#27892: refactor: Avoid copy of bilingual_str when formatting, Fix ADL violation

fa8ef7d refactor: Avoid copy of bilingual_str when formatting, Fix ADL violation (MarcoFalke)

Pull request description:

  This refactor shouldn't change behavior, but may fix compile errors such as bitcoin/bitcoin#27862 (comment)

ACKs for top commit:
  achow101:
    ACK fa8ef7d
  ryanofsky:
    Code review ACK fa8ef7d. Looks great! Thanks for updating
  hebasto:
    ACK fa8ef7d, I have reviewed the code and it looks OK.

Tree-SHA512: 903019962f27b5432b8e3af052b472238ef68d3ee165148c9d2232bf290309075f9f17d8d06c9b5c7fddb89c1a9c3a4c09c6310af01e8561adc0244a30db0857

Author: achow101

src/util/translation.h

@@ -49,22 +49,18 @@ inline bilingual_str Untranslated(std::string original) { return {original, orig
 
 // Provide an overload of tinyformat::format which can take bilingual_str arguments.
 namespace tinyformat {
-inline std::string TranslateArg(const bilingual_str& arg, bool translated)
-{
-    return translated ? arg.translated : arg.original;
-}
-
-template <typename T>
-inline T const& TranslateArg(const T& arg, bool translated)
-{
-    return arg;
-}
-
 template <typename... Args>
 bilingual_str format(const bilingual_str& fmt, const Args&... args)
 {
-    return bilingual_str{format(fmt.original, TranslateArg(args, false)...),
-                         format(fmt.translated, TranslateArg(args, true)...)};
+    const auto translate_arg{[](const auto& arg, bool translated) -> const auto& {
+        if constexpr (std::is_same_v<decltype(arg), const bilingual_str&>) {
+            return translated ? arg.translated : arg.original;
+        } else {
+            return arg;
+        }
+    }};
+    return bilingual_str{tfm::format(fmt.original, translate_arg(args, false)...),
+                         tfm::format(fmt.translated, translate_arg(args, true)...)};
 }
 } // namespace tinyformat