cmNinjaNormalTargetGenerator: Fix "Language" rule variable

Fixes: commit 3bc63e99e4 (Refactor: Prepare Ninja generator for multi-config, 2019-11-14, v3.17.0-rc1~290^2~3)

Source/cmNinjaNormalTargetGenerator.cxx

@@ -368,7 +368,7 @@ void cmNinjaNormalTargetGenerator::WriteLinkRule(bool useResponseFile,
     vars.CMTargetType = cmState::GetTargetTypeName(targetType).c_str();
 
     std::string lang = this->TargetLinkLanguage(config);
-    vars.Language = config.c_str();
+    vars.Language = lang.c_str();
     vars.AIXExports = "$AIX_EXPORTS";
 
     if (this->TargetLinkLanguage(config) == "Swift") {