diff --git a/library/common/src/main/java/com/google/android/exoplayer2/source/TrackGroup.java b/library/common/src/main/java/com/google/android/exoplayer2/source/TrackGroup.java
index 607f797103..2df780ce93 100644
--- a/library/common/src/main/java/com/google/android/exoplayer2/source/TrackGroup.java
+++ b/library/common/src/main/java/com/google/android/exoplayer2/source/TrackGroup.java
@@ -21,11 +21,14 @@ import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.Format;
 import com.google.android.exoplayer2.util.Assertions;
+import com.google.android.exoplayer2.util.Log;
 import java.util.Arrays;
 
 /** Defines an immutable group of tracks identified by their format identity. */
 public final class TrackGroup implements Parcelable {
 
+  private static final String TAG = "TrackGroup";
+
   /** The number of tracks in the group. */
   public final int length;
 
@@ -41,6 +44,7 @@ public final class TrackGroup implements Parcelable {
     Assertions.checkState(formats.length > 0);
     this.formats = formats;
     this.length = formats.length;
+    verifyCorrectness();
   }
 
   /* package */ TrackGroup(Parcel in) {
@@ -129,4 +133,62 @@ public final class TrackGroup implements Parcelable {
           return new TrackGroup[size];
         }
       };
+
+  private void verifyCorrectness() {
+    // TrackGroups should only contain tracks with exactly the same content (but in different
+    // qualities). We only log an error instead of throwing to not break backwards-compatibility for
+    // cases where malformed TrackGroups happen to work by chance (e.g. because adaptive selections
+    // are always disabled).
+    String language = normalizeLanguage(formats[0].language);
+    @C.RoleFlags int roleFlags = normalizeRoleFlags(formats[0].roleFlags);
+    for (int i = 1; i < formats.length; i++) {
+      if (!language.equals(normalizeLanguage(formats[i].language))) {
+        logErrorMessage(
+            /* mismatchField= */ "languages",
+            /* valueIndex0= */ formats[0].language,
+            /* otherValue=* */ formats[i].language,
+            /* otherIndex= */ i);
+        return;
+      }
+      if (roleFlags != normalizeRoleFlags(formats[i].roleFlags)) {
+        logErrorMessage(
+            /* mismatchField= */ "role flags",
+            /* valueIndex0= */ Integer.toBinaryString(formats[0].roleFlags),
+            /* otherValue=* */ Integer.toBinaryString(formats[i].roleFlags),
+            /* otherIndex= */ i);
+        return;
+      }
+    }
+  }
+
+  private static String normalizeLanguage(@Nullable String language) {
+    // Treat all variants of undetermined or unknown languages as compatible.
+    return language == null || language.equals(C.LANGUAGE_UNDETERMINED) ? "" : language;
+  }
+
+  @C.RoleFlags
+  private static int normalizeRoleFlags(@C.RoleFlags int roleFlags) {
+    // Treat trick-play and non-trick-play formats as compatible.
+    return roleFlags | C.ROLE_FLAG_TRICK_PLAY;
+  }
+
+  private static void logErrorMessage(
+      String mismatchField,
+      @Nullable String valueIndex0,
+      @Nullable String otherValue,
+      int otherIndex) {
+    Log.e(
+        TAG,
+        "",
+        new IllegalStateException(
+            "Different "
+                + mismatchField
+                + " combined in one TrackGroup: '"
+                + valueIndex0
+                + "' (track 0) and '"
+                + otherValue
+                + "' (track "
+                + otherIndex
+                + ")"));
+  }
 }
diff --git a/library/common/src/main/java/com/google/android/exoplayer2/util/Util.java b/library/common/src/main/java/com/google/android/exoplayer2/util/Util.java
index 0dbce9a70f..d04b4f8341 100644
--- a/library/common/src/main/java/com/google/android/exoplayer2/util/Util.java
+++ b/library/common/src/main/java/com/google/android/exoplayer2/util/Util.java
@@ -670,7 +670,7 @@ public final class Util {
     // Locale data (especially for API < 21) may produce tags with '_' instead of the
     // standard-conformant '-'.
     String normalizedTag = language.replace('_', '-');
-    if (normalizedTag.isEmpty() || "und".equals(normalizedTag)) {
+    if (normalizedTag.isEmpty() || normalizedTag.equals(C.LANGUAGE_UNDETERMINED)) {
       // Tag isn't valid, keep using the original.
       normalizedTag = language;
     }
diff --git a/library/extractor/src/main/java/com/google/android/exoplayer2/extractor/mp4/MetadataUtil.java b/library/extractor/src/main/java/com/google/android/exoplayer2/extractor/mp4/MetadataUtil.java
index 53ed281152..a5470dace6 100644
--- a/library/extractor/src/main/java/com/google/android/exoplayer2/extractor/mp4/MetadataUtil.java
+++ b/library/extractor/src/main/java/com/google/android/exoplayer2/extractor/mp4/MetadataUtil.java
@@ -279,8 +279,6 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
         "Psybient"
       };
 
-  private static final String LANGUAGE_UNDEFINED = "und";
-
   private static final int TYPE_TOP_BYTE_COPYRIGHT = 0xA9;
   private static final int TYPE_TOP_BYTE_REPLACEMENT = 0xFD; // Truncated value of \uFFFD.
 
@@ -467,7 +465,7 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
     if (atomType == Atom.TYPE_data) {
       data.skipBytes(8); // version (1), flags (3), empty (4)
       String value = data.readNullTerminatedString(atomSize - 16);
-      return new CommentFrame(LANGUAGE_UNDEFINED, value, value);
+      return new CommentFrame(C.LANGUAGE_UNDETERMINED, value, value);
     }
     Log.w(TAG, "Failed to parse comment attribute: " + Atom.getAtomTypeString(type));
     return null;
@@ -487,7 +485,7 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
     if (value >= 0) {
       return isTextInformationFrame
           ? new TextInformationFrame(id, /* description= */ null, Integer.toString(value))
-          : new CommentFrame(LANGUAGE_UNDEFINED, id, Integer.toString(value));
+          : new CommentFrame(C.LANGUAGE_UNDETERMINED, id, Integer.toString(value));
     }
     Log.w(TAG, "Failed to parse uint8 attribute: " + Atom.getAtomTypeString(type));
     return null;