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-34488

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

@@ -641,4 +641,12 @@ class MockHttpServletResponseTests {
 		assertThat(response.getContentAsString()).isEqualTo(content);
 	}
 
+	@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);
+	}
+
 }