Index: /trunk/src/org/openstreetmap/josm/data/validation/tests/TagChecker.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/validation/tests/TagChecker.java	(revision 15887)
+++ /trunk/src/org/openstreetmap/josm/data/validation/tests/TagChecker.java	(revision 15888)
@@ -466,5 +466,7 @@
 
     private static boolean isAllowedPhoneticCharacter(String key, int c) {
-        return c == 0x0259                                          // U+0259 is used as a standard character in azerbaidjani
+        return c == 0x0259 || c == 0x018F // U+0259 is paired with the capital letter U+018F in Azeri, see #18740
+            || c == 0x0254 || c == 0x0186 // U+0254 is paired with the capital letter U+0186 in several African languages, see #18740
+            || c == 0x025B || c == 0x0190 // U+025B is paired with the capital letter U+0190 in several African languages, see #18740
             || (key.endsWith("ref") && 0x1D2C <= c && c <= 0x1D42); // allow uppercase superscript latin characters in *ref tags
     }
Index: /trunk/test/unit/org/openstreetmap/josm/data/validation/tests/TagCheckerTest.java
===================================================================
--- /trunk/test/unit/org/openstreetmap/josm/data/validation/tests/TagCheckerTest.java	(revision 15887)
+++ /trunk/test/unit/org/openstreetmap/josm/data/validation/tests/TagCheckerTest.java	(revision 15888)
@@ -343,4 +343,12 @@
 
     /**
+     * Non-regression test for <a href="https://josm.openstreetmap.de/ticket/18740">Bug #18740</a>.
+     */
+    @Test
+    public void testTicket18740() {
+        assertFalse(TagChecker.containsUnusualUnicodeCharacter("name:ak", "Frɛnkyeman"));
+    }
+
+    /**
      * Detects objects with types not supported by their presets.
      * @throws IOException in case of I/O error