diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/Session.java b/acme4j-client/src/main/java/org/shredzone/acme4j/Session.java
index 6d2db56c..3ffd51eb 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/Session.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/Session.java
@@ -32,6 +32,7 @@ import org.shredzone.acme4j.connector.Resource;
 import org.shredzone.acme4j.exception.AcmeException;
 import org.shredzone.acme4j.provider.AcmeProvider;
 import org.shredzone.acme4j.provider.GenericAcmeProvider;
+import org.shredzone.acme4j.toolbox.AcmeUtils;
 import org.shredzone.acme4j.toolbox.JSON;
 import org.shredzone.acme4j.toolbox.JSON.Value;
 
@@ -49,7 +50,8 @@ public class Session {
     private final AcmeProvider provider;
 
     private @Nullable String nonce;
-    private Locale locale = Locale.getDefault();
+    private @Nullable Locale locale = Locale.getDefault();
+    private String languageHeader = AcmeUtils.localeToLanguageHeader(Locale.getDefault());
     protected @Nullable ZonedDateTime directoryLastModified;
     protected @Nullable ZonedDateTime directoryExpires;
 
@@ -147,8 +149,10 @@ public class Session {
     }
 
     /**
-     * Gets the current locale of this session.
+     * Gets the current locale of this session, or {@code null} if no special language is
+     * selected.
      */
+    @Nullable
     public Locale getLocale() {
         return locale;
     }
@@ -156,9 +160,21 @@ public class Session {
     /**
      * Sets the locale used in this session. The locale is passed to the server as
      * Accept-Language header. The server <em>may</em> respond with localized messages.
+     * The default is the system's language. If set to {@code null}, no special language
+     * is selected.
      */
     public void setLocale(@Nullable Locale locale) {
-        this.locale = locale != null ? locale : Locale.getDefault();
+        this.locale = locale;
+        this.languageHeader = AcmeUtils.localeToLanguageHeader(locale);
+    }
+
+    /**
+     * Gets an Accept-Language header value that matches the current locale.
+     *
+     * @since 3.0.0
+     */
+    public String getLanguageHeader() {
+        return languageHeader;
     }
 
     /**
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java b/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
index 96f8bae2..646231cd 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
@@ -347,7 +347,7 @@ public class DefaultConnection implements Connection {
         try {
             var builder = httpConnector.createRequestBuilder(url)
                     .header(ACCEPT_CHARSET_HEADER, DEFAULT_CHARSET)
-                    .header(ACCEPT_LANGUAGE_HEADER, session.getLocale().toLanguageTag());
+                    .header(ACCEPT_LANGUAGE_HEADER, session.getLanguageHeader());
 
             if (session.networkSettings().isCompressionEnabled()) {
                 builder.header(ACCEPT_ENCODING_HEADER, "gzip");
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
index ccbf8747..9752bd66 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
@@ -26,6 +26,7 @@ import java.time.Instant;
 import java.time.ZoneId;
 import java.time.ZonedDateTime;
 import java.util.Base64;
+import java.util.Locale;
 import java.util.Objects;
 import java.util.regex.Pattern;
 
@@ -221,6 +222,29 @@ public final class AcmeUtils {
                 ZoneId.of(tz)).toInstant();
     }
 
+    /**
+     * Converts the given locale to an Accept-Language header value.
+     *
+     * @param locale
+     *         {@link Locale} to be used in the header
+     * @return Value that can be used in an Accept-Language header
+     */
+    public static String localeToLanguageHeader(@Nullable Locale locale) {
+        if (locale == null || "und".equals(locale.toLanguageTag())) {
+            return "*";
+        }
+
+        var langTag = locale.toLanguageTag();
+
+        var header = new StringBuilder(langTag);
+        if (langTag.indexOf('-') >= 0) {
+            header.append(',').append(locale.getLanguage()).append(";q=0.8");
+        }
+        header.append(",*;q=0.1");
+
+        return header.toString();
+    }
+
     /**
      * Strips the acme error prefix from the error string.
      * <p>
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/SessionTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/SessionTest.java
index 4a857413..88f2d568 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/SessionTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/SessionTest.java
@@ -23,6 +23,7 @@ import java.net.URI;
 import java.net.URL;
 import java.time.Duration;
 import java.time.ZonedDateTime;
+import java.util.Locale;
 
 import org.assertj.core.api.AutoCloseableSoftAssertions;
 import org.junit.jupiter.api.Test;
@@ -31,6 +32,7 @@ import org.shredzone.acme4j.connector.Resource;
 import org.shredzone.acme4j.exception.AcmeException;
 import org.shredzone.acme4j.provider.AcmeProvider;
 import org.shredzone.acme4j.provider.GenericAcmeProvider;
+import org.shredzone.acme4j.toolbox.AcmeUtils;
 import org.shredzone.acme4j.toolbox.TestUtils;
 
 /**
@@ -212,4 +214,28 @@ public class SessionTest {
         }
     }
 
+    /**
+     * Test that the locale is properly set.
+     */
+    @Test
+    public void testLocale() {
+        var session = new Session(URI.create(TestUtils.ACME_SERVER_URI));
+
+        // default configuration
+        assertThat(session.getLocale())
+                .isEqualTo(Locale.getDefault());
+        assertThat(session.getLanguageHeader())
+                .isEqualTo(AcmeUtils.localeToLanguageHeader(Locale.getDefault()));
+
+        // null
+        session.setLocale(null);
+        assertThat(session.getLocale()).isNull();
+        assertThat(session.getLanguageHeader()).isEqualTo("*");
+
+        // a locale
+        session.setLocale(Locale.CANADA_FRENCH);
+        assertThat(session.getLocale()).isEqualTo(Locale.CANADA_FRENCH);
+        assertThat(session.getLanguageHeader()).isEqualTo("fr-CA,fr;q=0.8,*;q=0.1");
+    }
+
 }
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/connector/DefaultConnectionTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/connector/DefaultConnectionTest.java
index 80a84472..60fb4f32 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/connector/DefaultConnectionTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/connector/DefaultConnectionTest.java
@@ -72,6 +72,9 @@ public class DefaultConnectionTest {
     private static final String DIRECTORY_PATH = "/dir";
     private static final String NEW_NONCE_PATH = "/newNonce";
     private static final String REQUEST_PATH = "/test/test";
+    private static final String TEST_ACCEPT_LANGUAGE = "ja-JP,ja;q=0.8,*;q=0.1";
+    private static final String TEST_ACCEPT_CHARSET = "utf-8";
+    private static final String TEST_USER_AGENT_PATTERN = "^acme4j/.*$";
 
     private final URL accountUrl = TestUtils.url(TestUtils.ACCOUNT_URL);
     private Session session;
@@ -564,9 +567,9 @@ public class DefaultConnectionTest {
 
         verify(getRequestedFor(urlEqualTo(REQUEST_PATH))
                 .withHeader("Accept", equalTo("application/json"))
-                .withHeader("Accept-Charset", equalTo("utf-8"))
-                .withHeader("Accept-Language", equalTo("ja-JP"))
-                .withHeader("User-Agent", matching("^acme4j/.*$"))
+                .withHeader("Accept-Charset", equalTo(TEST_ACCEPT_CHARSET))
+                .withHeader("Accept-Language", equalTo(TEST_ACCEPT_LANGUAGE))
+                .withHeader("User-Agent", matching(TEST_USER_AGENT_PATTERN))
         );
     }
 
@@ -589,9 +592,9 @@ public class DefaultConnectionTest {
         verify(getRequestedFor(urlEqualTo(REQUEST_PATH))
                 .withHeader("If-Modified-Since", equalToDateTime(ifModifiedSince))
                 .withHeader("Accept", equalTo("application/json"))
-                .withHeader("Accept-Charset", equalTo("utf-8"))
-                .withHeader("Accept-Language", equalTo("ja-JP"))
-                .withHeader("User-Agent", matching("^acme4j/.*$"))
+                .withHeader("Accept-Charset", equalTo(TEST_ACCEPT_CHARSET))
+                .withHeader("Accept-Language", equalTo(TEST_ACCEPT_LANGUAGE))
+                .withHeader("User-Agent", matching(TEST_USER_AGENT_PATTERN))
         );
     }
 
@@ -620,9 +623,9 @@ public class DefaultConnectionTest {
 
         verify(postRequestedFor(urlEqualTo(REQUEST_PATH))
                 .withHeader("Accept", equalTo("application/json"))
-                .withHeader("Accept-Charset", equalTo("utf-8"))
-                .withHeader("Accept-Language", equalTo("ja-JP"))
-                .withHeader("User-Agent", matching("^acme4j/.*$"))
+                .withHeader("Accept-Charset", equalTo(TEST_ACCEPT_CHARSET))
+                .withHeader("Accept-Language", equalTo(TEST_ACCEPT_LANGUAGE))
+                .withHeader("User-Agent", matching(TEST_USER_AGENT_PATTERN))
         );
 
         var requests = findAll(postRequestedFor(urlEqualTo(REQUEST_PATH)));
@@ -673,10 +676,10 @@ public class DefaultConnectionTest {
 
         verify(postRequestedFor(urlEqualTo(REQUEST_PATH))
                 .withHeader("Accept", equalTo("application/json"))
-                .withHeader("Accept-Charset", equalTo("utf-8"))
-                .withHeader("Accept-Language", equalTo("ja-JP"))
+                .withHeader("Accept-Charset", equalTo(TEST_ACCEPT_CHARSET))
+                .withHeader("Accept-Language", equalTo(TEST_ACCEPT_LANGUAGE))
                 .withHeader("Content-Type", equalTo("application/jose+json"))
-                .withHeader("User-Agent", matching("^acme4j/.*$"))
+                .withHeader("User-Agent", matching(TEST_USER_AGENT_PATTERN))
         );
 
         var requests = findAll(postRequestedFor(urlEqualTo(REQUEST_PATH)));
@@ -727,10 +730,10 @@ public class DefaultConnectionTest {
 
         verify(postRequestedFor(urlEqualTo(REQUEST_PATH))
                 .withHeader("Accept", equalTo("application/pem-certificate-chain"))
-                .withHeader("Accept-Charset", equalTo("utf-8"))
-                .withHeader("Accept-Language", equalTo("ja-JP"))
+                .withHeader("Accept-Charset", equalTo(TEST_ACCEPT_CHARSET))
+                .withHeader("Accept-Language", equalTo(TEST_ACCEPT_LANGUAGE))
                 .withHeader("Content-Type", equalTo("application/jose+json"))
-                .withHeader("User-Agent", matching("^acme4j/.*$"))
+                .withHeader("User-Agent", matching(TEST_USER_AGENT_PATTERN))
         );
     }
 
@@ -758,10 +761,10 @@ public class DefaultConnectionTest {
 
         verify(postRequestedFor(urlEqualTo(REQUEST_PATH))
                 .withHeader("Accept", equalTo("application/json"))
-                .withHeader("Accept-Charset", equalTo("utf-8"))
-                .withHeader("Accept-Language", equalTo("ja-JP"))
+                .withHeader("Accept-Charset", equalTo(TEST_ACCEPT_CHARSET))
+                .withHeader("Accept-Language", equalTo(TEST_ACCEPT_LANGUAGE))
                 .withHeader("Content-Type", equalTo("application/jose+json"))
-                .withHeader("User-Agent", matching("^acme4j/.*$"))
+                .withHeader("User-Agent", matching(TEST_USER_AGENT_PATTERN))
         );
 
         var requests = findAll(postRequestedFor(urlEqualTo(REQUEST_PATH)));
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
index 383c311a..12d4c0c7 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
@@ -27,6 +27,7 @@ import java.net.URI;
 import java.security.Security;
 import java.security.cert.CertificateEncodingException;
 import java.time.temporal.ChronoUnit;
+import java.util.Locale;
 import java.util.stream.Stream;
 
 import org.bouncycastle.jce.provider.BouncyCastleProvider;
@@ -201,6 +202,25 @@ public class AcmeUtilsTest {
                 "accepted string without time");
     }
 
+    /**
+     * Test that locales are correctly converted to language headers.
+     */
+    @Test
+    public void testLocaleToLanguageHeader() {
+        assertThat(localeToLanguageHeader(Locale.ENGLISH))
+                .isEqualTo("en,*;q=0.1");
+        assertThat(localeToLanguageHeader(new Locale("en", "US")))
+                .isEqualTo("en-US,en;q=0.8,*;q=0.1");
+        assertThat(localeToLanguageHeader(Locale.GERMAN))
+                .isEqualTo("de,*;q=0.1");
+        assertThat(localeToLanguageHeader(Locale.GERMANY))
+                .isEqualTo("de-DE,de;q=0.8,*;q=0.1");
+        assertThat(localeToLanguageHeader(new Locale("")))
+                .isEqualTo("*");
+        assertThat(localeToLanguageHeader(null))
+                .isEqualTo("*");
+    }
+
     /**
      * Test that error prefix is correctly removed.
      */