diff --git a/CHANGELOG.md b/CHANGELOG.md
index da8df871..6ced8c11 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,16 @@ FlatLaf Change Log
 - Tree and List: Fixed painting of rounded drop backgrounds. (issue #1023)
 
 
+#### Incompatibilities
+
+- FlatLaf properties files are now loaded using the UTF-8 character encoding
+  instead of ISO 8859-1. In usual properties files you will not notice any
+  difference because they use only ASCII characters, but if you've put localized
+  (non-English) texts (e.g. German umlauts) into your properties files, you need
+  to convert them to UTF-8. Properties files created with the FlatLaf Theme
+  Editor already use UTF-8, including in older versions. (issue #1031)
+
+
 ## 3.6.1
 
 - Extras: Support JSVG 2.0.0. Minimum JSVG version is now 1.6.0. (issue #997)
diff --git a/flatlaf-core/src/main/java/com/formdev/flatlaf/FlatPropertiesLaf.java b/flatlaf-core/src/main/java/com/formdev/flatlaf/FlatPropertiesLaf.java
index d16d1b96..36e3580e 100644
--- a/flatlaf-core/src/main/java/com/formdev/flatlaf/FlatPropertiesLaf.java
+++ b/flatlaf-core/src/main/java/com/formdev/flatlaf/FlatPropertiesLaf.java
@@ -20,6 +20,9 @@ import java.io.File;
 import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Locale;
 import java.util.Properties;
@@ -62,8 +65,8 @@ public class FlatPropertiesLaf
 		throws IOException
 	{
 		Properties properties = new Properties();
-		try( InputStream in2 = in ) {
-			properties.load( in2 );
+		try( Reader reader = new InputStreamReader( in, StandardCharsets.UTF_8 )) {
+			properties.load( reader );
 		}
 		return properties;
 	}
diff --git a/flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java b/flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java
index a2e614ec..b13906b5 100644
--- a/flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java
+++ b/flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java
@@ -25,12 +25,15 @@ import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.Reader;
 import java.io.StreamTokenizer;
 import java.io.StringReader;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.Executable;
 import java.lang.reflect.Method;
 import java.net.URL;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -112,6 +115,14 @@ class UIDefaultsLoader
 		Set<String> specialPrefixes = FlatLaf.getUIKeySpecialPrefixes();
 
 		return new Properties() {
+			@Override
+			public void load( InputStream in ) throws IOException {
+				// use UTF-8 to load properties file
+				try( Reader reader = new InputStreamReader( in, StandardCharsets.UTF_8 )) {
+					super.load( reader );
+				}
+			}
+
 			@Override
 			public synchronized Object put( Object k, Object value ) {
 				// process key prefixes (while loading properties files)
diff --git a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatCompletionProvider.java b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatCompletionProvider.java
index 0bb400fb..d0395e1a 100644
--- a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatCompletionProvider.java
+++ b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatCompletionProvider.java
@@ -22,6 +22,7 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.io.StringReader;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashSet;
@@ -177,7 +178,7 @@ class FlatCompletionProvider
 			try {
 				try( InputStream in = getClass().getResourceAsStream( "/com/formdev/flatlaf/themeeditor/FlatLafUIKeys.txt" ) ) {
 					if( in != null ) {
-						try( BufferedReader reader = new BufferedReader( new InputStreamReader( in, "UTF-8" ) ) ) {
+						try( BufferedReader reader = new BufferedReader( new InputStreamReader( in, StandardCharsets.UTF_8 ) ) ) {
 							String key;
 							while( (key = reader.readLine()) != null ) {
 								if( !isIgnored( key ) )
diff --git a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorPane.java b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorPane.java
index b256318b..e67972aa 100644
--- a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorPane.java
+++ b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorPane.java
@@ -23,6 +23,7 @@ import java.awt.Window;
 import java.awt.event.ActionEvent;
 import java.io.File;
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import javax.swing.Action;
 import javax.swing.BorderFactory;
 import javax.swing.InputMap;
@@ -211,7 +212,7 @@ class FlatThemeEditorPane
 	void load( File file ) throws IOException {
 		this.file = file;
 
-		textArea.load( FileLocation.create( file ), "UTF-8" );
+		textArea.load( FileLocation.create( file ), StandardCharsets.UTF_8 );
 	}
 
 	boolean reloadIfNecessary() {
diff --git a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeFileEditor.java b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeFileEditor.java
index 9340627a..d41267cd 100644
--- a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeFileEditor.java
+++ b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeFileEditor.java
@@ -39,6 +39,7 @@ import java.io.OutputStreamWriter;
 import java.io.Writer;
 import java.net.URI;
 import java.net.URISyntaxException;
+import java.nio.charset.StandardCharsets;
 import java.time.Year;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -675,7 +676,7 @@ class FlatThemeFileEditor
 	{
 		try(
 			FileOutputStream out = new FileOutputStream( file );
-			Writer writer = new OutputStreamWriter( out, "UTF-8" );
+			Writer writer = new OutputStreamWriter( out, StandardCharsets.UTF_8 );
 		) {
 			writer.write( content );
 		}
diff --git a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemePropertiesBaseManager.java b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemePropertiesBaseManager.java
index ad2e0cfd..2f259b2b 100644
--- a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemePropertiesBaseManager.java
+++ b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemePropertiesBaseManager.java
@@ -19,6 +19,9 @@ package com.formdev.flatlaf.themeeditor;
 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -109,8 +112,11 @@ class FlatThemePropertiesBaseManager
 			String propertiesName = '/' + lafClass.getName().replace( '.', '/' ) + ".properties";
 			try( InputStream in = lafClass.getResourceAsStream( propertiesName ) ) {
 				Properties properties = new Properties();
-				if( in != null )
-					properties.load( in );
+				if( in != null ) {
+					try( Reader reader = new InputStreamReader( in, StandardCharsets.UTF_8 )) {
+						properties.load( in );
+					}
+				}
 				coreThemes.put( lafClass.getSimpleName(), properties );
 			} catch( IOException ex ) {
 				ex.printStackTrace();