Fix multiple Content-Language values in MockHttpServletResponse

Prior to this commit, `MockHttpServletResponse` would only support
adding a `Content-Language` once. Adding multiple header values would
always replace the content-language property in the response and the
entire header value.

This commit ensures that this behavior is supported.

Fixes gh-34491

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -728,14 +728,17 @@ else if (HttpHeaders.CONTENT_LENGTH.equalsIgnoreCase(name)) {
 		}
 		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)) {

spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -631,4 +631,12 @@ void resetResetsCharset() {
 		assertThat(contentTypeHeader).isEqualTo("text/plain");
 	}
 
+	@Test // gh-34488
+	void shouldAddMultipleContentLanguage() {
+		response.addHeader("Content-Language", "en");
+		response.addHeader("Content-Language", "fr");
+		assertThat(response.getHeaders("Content-Language")).contains("en", "fr");
+		assertThat(response.getLocale()).isEqualTo(Locale.ENGLISH);
+	}
+
 }

spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -728,14 +728,17 @@ else if (HttpHeaders.CONTENT_LENGTH.equalsIgnoreCase(name)) {
 		}
 		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)) {