diff --git a/NEWS.md b/NEWS.md
index 57346e03a..a04ebb827 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -5,6 +5,9 @@
 * MKVToolNix GUI: multiplexer: deriving track languages from file names: the
   default list of boundary characters now includes `-`. Part of the
   implementation of #3173.
+* MKVToolNix GUI: multiplexer: deriving track languages from file names: the
+  GUI can now detect full BCP 47/RFC 5646 language tags in file names. Part of
+  the implementation of #3173.
 
 ## Build system changes
 
diff --git a/src/mkvtoolnix-gui/merge/source_file.cpp b/src/mkvtoolnix-gui/merge/source_file.cpp
index bc2574b5d..b698fbf00 100644
--- a/src/mkvtoolnix-gui/merge/source_file.cpp
+++ b/src/mkvtoolnix-gui/merge/source_file.cpp
@@ -369,6 +369,42 @@ SourceFile::deriveLanguageFromFileName() {
 
   QStringList escapedChars;
 
+  // First try to detect full BCP 47 language tags if they contain at
+  // least two components & don't start with x-. As BCP 47 tags
+  // contain '-' characters, don't split by them.
+  for (auto c : cfg.m_boundaryCharsForDerivingTrackLanguagesFromFileNames)
+    if (c != L'-')
+      escapedChars << QRegularExpression::escape(c);
+
+  QRegularExpression bcp47Re{Q("^[^x][a-z]+-"), QRegularExpression::CaseInsensitiveOption};
+
+  if (!escapedChars.isEmpty()) {
+    auto splitRE     = QRegularExpression{Q("(?:%1)+").arg(escapedChars.join(Q("|")))};
+    auto allCaptures = fileName.split(splitRE);
+
+    for (auto captureItr = allCaptures.rbegin(), captureEnd = allCaptures.rend(); captureItr != captureEnd; ++captureItr) {
+      auto &capture = *captureItr;
+
+      if (capture.isEmpty())
+        continue;
+
+      qDebug() << "language derivation match (BCP 47):" << capture;
+
+      if (!capture.contains(bcp47Re))
+        continue;
+
+      auto tag = mtx::bcp47::language_c::parse(to_utf8(capture));
+      if (tag.is_valid()) {
+        qDebug() << "derived BCP 47 language tag";
+        return tag;
+      }
+    }
+  }
+
+  // No full BCP 47 language tag found. Now look for languages only
+  // with the full set of boundary characters.
+  escapedChars.clear();
+
   for (auto c : cfg.m_boundaryCharsForDerivingTrackLanguagesFromFileNames)
     escapedChars << QRegularExpression::escape(c);