Index: trunk/test/unit/org/openstreetmap/josm/gui/bugreport/BugReportDialogTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/gui/bugreport/BugReportDialogTest.java	(revision 19637)
+++ trunk/test/unit/org/openstreetmap/josm/gui/bugreport/BugReportDialogTest.java	(revision 19637)
@@ -0,0 +1,52 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.gui.bugreport;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.nio.file.InvalidPathException;
+import java.util.Collections;
+
+import javax.swing.JOptionPane;
+
+import org.junit.jupiter.api.Test;
+import org.openstreetmap.josm.TestUtils;
+import org.openstreetmap.josm.testutils.mockers.JOptionPaneSimpleMocker;
+import org.openstreetmap.josm.tools.bugreport.BugReport;
+import org.openstreetmap.josm.tools.bugreport.BugReportQueue.SuppressionMode;
+import org.openstreetmap.josm.tools.bugreport.ReportedException;
+
+/**
+ * Unit tests of {@link BugReportDialog}.
+ */
+class BugReportDialogTest {
+
+    /**
+     * A file name the locale cannot represent is explained to the user, once, instead of asking for a bug report.
+     * See #14596.
+     */
+    @Test
+    void testUnmappableFileNameIsExplainedOnce() {
+        TestUtils.assumeWorkingJMockit();
+        final JOptionPaneSimpleMocker mocker = new JOptionPaneSimpleMocker(Collections.singletonMap(
+                "<html>JOSM cannot handle the name of a file, because it is running with the character set ANSI_X3.4-1968 "
+                        + "for file names, which cannot represent it.<br>"
+                        + "This happens when JOSM is started without a UTF-8 locale, for example with <tt>LANG=C</tt>, "
+                        + "or with a locale which is not installed.<br><br>"
+                        + "Please start JOSM with the environment variable <tt>LC_ALL=C.UTF-8</tt>.</html>",
+                JOptionPane.OK_OPTION));
+        final ReportedException e = BugReport.intercept(new InvalidPathException("/home/user/Capture d'écran.png",
+                "Malformed input or input contains unmappable characters"));
+        final String jnuEncoding = System.getProperty("sun.jnu.encoding");
+        try {
+            System.setProperty("sun.jnu.encoding", "ANSI_X3.4-1968");
+            // no bug report dialog: that would fail here, as the tests run headless
+            assertEquals(SuppressionMode.NONE, BugReportDialog.showFor(e, 0));
+            // browsing another folder throws it again, which is not worth another message
+            assertEquals(SuppressionMode.NONE, BugReportDialog.showFor(e, 1));
+        } finally {
+            System.setProperty("sun.jnu.encoding", jnuEncoding);
+        }
+        assertEquals(1, mocker.getInvocationLog().size());
+        assertEquals("Unsupported file name", mocker.getInvocationLog().get(0)[2]);
+    }
+}
Index: trunk/test/unit/org/openstreetmap/josm/tools/bugreport/ReportedExceptionTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/tools/bugreport/ReportedExceptionTest.java	(revision 19629)
+++ trunk/test/unit/org/openstreetmap/josm/tools/bugreport/ReportedExceptionTest.java	(revision 19637)
@@ -3,5 +3,8 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.nio.file.InvalidPathException;
 import java.util.Arrays;
 
@@ -62,4 +65,29 @@
 
     /**
+     * Tests {@link ReportedException#isUnmappableFileName()}: a file name which the character set used for file names
+     * cannot represent is recognised, also as the cause of another exception, but only if that character set is not
+     * UTF-8 - otherwise the locale is not to blame. See #14596.
+     */
+    @Test
+    void testIsUnmappableFileName() {
+        // the exception sun.nio.fs.UnixPath throws for "Capture d'écran.png" when the locale is C or not installed
+        InvalidPathException unmappable = new InvalidPathException("/home/user/Capture d'\u00e9cran.png",
+                "Malformed input or input contains unmappable characters");
+        String jnuEncoding = System.getProperty("sun.jnu.encoding");
+        try {
+            System.setProperty("sun.jnu.encoding", "ANSI_X3.4-1968");
+            assertTrue(new ReportedException(unmappable).isUnmappableFileName());
+            assertTrue(new ReportedException(new IllegalStateException(unmappable)).isUnmappableFileName());
+            assertFalse(new ReportedException(new InvalidPathException("a\0b", "Nul character not allowed")).isUnmappableFileName());
+            assertFalse(new ReportedException(new IllegalStateException()).isUnmappableFileName());
+
+            System.setProperty("sun.jnu.encoding", "UTF-8");
+            assertFalse(new ReportedException(unmappable).isUnmappableFileName());
+        } finally {
+            System.setProperty("sun.jnu.encoding", jnuEncoding);
+        }
+    }
+
+    /**
      * Tests that {@link ReportedException#isSame(ReportedException)} works as expected.
      */