Extend bilingual_str support for tinyformat

Previous bilingual_str tinyformat::format accepted bilingual format strings,
but not bilingual arguments. Extend it to accept both. This is useful when
embedding one translated string inside another translated string, for example:
`strprintf(_("Error: %s"), message)` which would fail previously if `message`
was a bilingual_str.

src/Makefile.test.include

@@ -147,6 +147,7 @@ BITCOIN_TESTS =\
   test/timedata_tests.cpp \
   test/torcontrol_tests.cpp \
   test/transaction_tests.cpp \
+  test/translation_tests.cpp \
   test/txindex_tests.cpp \
   test/txpackage_tests.cpp \
   test/txreconciliation_tests.cpp \

src/test/translation_tests.cpp

@@ -0,0 +1,21 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <tinyformat.h>
+#include <util/translation.h>
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_AUTO_TEST_SUITE(translation_tests)
+
+BOOST_AUTO_TEST_CASE(translation_namedparams)
+{
+    bilingual_str arg{"original", "translated"};
+    bilingual_str format{"original [%s]", "translated [%s]"};
+    bilingual_str result{strprintf(format, arg)};
+    BOOST_CHECK_EQUAL(result.original, "original [original]");
+    BOOST_CHECK_EQUAL(result.translated, "translated [translated]");
+}
+
+BOOST_AUTO_TEST_SUITE_END()

src/util/translation.h

@@ -47,11 +47,24 @@ inline bilingual_str operator+(bilingual_str lhs, const bilingual_str& rhs)
 /** Mark a bilingual_str as untranslated */
 inline bilingual_str Untranslated(std::string original) { return {original, original}; }
 
+// 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, args...), format(fmt.translated, args...)};
+    return bilingual_str{format(fmt.original, TranslateArg(args, false)...),
+                         format(fmt.translated, TranslateArg(args, true)...)};
 }
 } // namespace tinyformat