diff --git a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
index 42c0586368..95bc2551b8 100644
--- a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
+++ b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
@@ -755,14 +755,17 @@ public class MockHttpServletResponse implements HttpServletResponse {
 		}
 		else if (HttpHeaders.CONTENT_LANGUAGE.equalsIgnoreCase(name)) {
 			String contentLanguages = value.toString();
-			HttpHeaders headers = new HttpHeaders();
-			headers.add(HttpHeaders.CONTENT_LANGUAGE, contentLanguages);
-			Locale language = headers.getContentLanguage();
-			setLocale(language != null ? language : Locale.getDefault());
-			// Since setLocale() sets the Content-Language header to the given
-			// single Locale, we have to explicitly set the Content-Language header
-			// to the user-provided value.
-			doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, contentLanguages, true);
+			// only set the locale if we replace the header or if there was none before
+			if (replaceHeader || !this.headers.containsKey(HttpHeaders.CONTENT_LANGUAGE)) {
+				HttpHeaders headers = new HttpHeaders();
+				headers.add(HttpHeaders.CONTENT_LANGUAGE, contentLanguages);
+				Locale language = headers.getContentLanguage();
+				this.locale = language != null ? language : Locale.getDefault();
+				doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, contentLanguages, replaceHeader);
+			}
+			else {
+				doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, contentLanguages, false);
+			}
 			return true;
 		}
 		else if (HttpHeaders.SET_COOKIE.equalsIgnoreCase(name)) {
diff --git a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
index 5982639b23..069f38cf89 100644
--- a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
+++ b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
@@ -290,6 +290,14 @@ class MockHttpServletResponseTests {
 			assertThat(response.getLocale()).isEqualTo(Locale.ITALIAN);
 		}
 
+		@Test // gh-34488
+		void shouldAddMultipleContentLanguage() {
+			response.addHeader(HttpHeaders.CONTENT_LANGUAGE, "en");
+			response.addHeader(HttpHeaders.CONTENT_LANGUAGE, "fr");
+			assertThat(response.getHeaders(HttpHeaders.CONTENT_LANGUAGE)).contains("en", "fr");
+			assertThat(response.getLocale()).isEqualTo(Locale.ENGLISH);
+		}
+
 		@Test
 		void contentLengthSetsHeader() {
 			response.setContentLength(66);
@@ -672,5 +680,4 @@ class MockHttpServletResponseTests {
 
 	}
 
-
 }
diff --git a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
index ea204211e5..9519dafc9d 100644
--- a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
+++ b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
@@ -755,14 +755,17 @@ public class MockHttpServletResponse implements HttpServletResponse {
 		}
 		else if (HttpHeaders.CONTENT_LANGUAGE.equalsIgnoreCase(name)) {
 			String contentLanguages = value.toString();
-			HttpHeaders headers = new HttpHeaders();
-			headers.add(HttpHeaders.CONTENT_LANGUAGE, contentLanguages);
-			Locale language = headers.getContentLanguage();
-			setLocale(language != null ? language : Locale.getDefault());
-			// Since setLocale() sets the Content-Language header to the given
-			// single Locale, we have to explicitly set the Content-Language header
-			// to the user-provided value.
-			doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, contentLanguages, true);
+			// only set the locale if we replace the header or if there was none before
+			if (replaceHeader || !this.headers.containsKey(HttpHeaders.CONTENT_LANGUAGE)) {
+				HttpHeaders headers = new HttpHeaders();
+				headers.add(HttpHeaders.CONTENT_LANGUAGE, contentLanguages);
+				Locale language = headers.getContentLanguage();
+				this.locale = language != null ? language : Locale.getDefault();
+				doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, contentLanguages, replaceHeader);
+			}
+			else {
+				doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, contentLanguages, false);
+			}
 			return true;
 		}
 		else if (HttpHeaders.SET_COOKIE.equalsIgnoreCase(name)) {