Index: wp-testcase/test_includes_formatting.php
===================================================================
--- wp-testcase/test_includes_formatting.php	(revision 445)
+++ wp-testcase/test_includes_formatting.php	(working copy)
@@ -1164,4 +1164,31 @@
 	}
 }
 
+class TestRemoveAccents extends WPTestCase {
+	// #17738
+	public function test_replaces_vowels_diacritic() {
+		$this->assertEquals( 'OoUu', remove_accents( 'ƠơƯư' ), 'remove_accents replaces vowels with diacritic' );
+	}
+
+	public function test_replaces_grave_accent() {
+		$this->assertEquals( 'AaAaEeOoOoUuYy', remove_accents( 'ẦầẰằỀềỒồỜờỪừỲỳ' ), 'remove_accents replaces grave diacritic' );
+	}
+
+	public function test_replaces_hook() {
+		$this->assertEquals( 'AaAaAaEeEeIiOoOoOoUuUuYy', remove_accents( 'ẢảẨẩẲẳẺẻỂểỈỉỎỏỔổỞởỦủỬửỶỷ' ), 'remove_accents replaces hook diacritic' );
+	}
+
+	public function test_replaces_tilde() {
+		$this->assertEquals( 'AaAaEeEeOoOoUuYy', remove_accents( 'ẪẫẴẵẼẽỄễỖỗỠỡỮữỸỹ' ), 'remove_accents replaces tilde diacritic' );
+	}
+
+	public function test_replaces_acute() {
+		$this->assertEquals( 'AaAaEeOoOoUu', remove_accents( 'ẤấẮắẾếỐốỚớỨứ' ), 'remove_accents replaces accute accent' );
+	}
+
+	public function test_replaces_dot_below() {
+		$this->assertEquals( 'AaAaAaEeEeIiOoOoOoUuUuYy', remove_accents( 'ẠạẬậẶặẸẹỆệỊịỌọỘộỢợỤụỰựỴỵ' ), 'remove_accents replaces dot below' );
+	}
+}
+
 ?>