diff --git a/src/server/i18n.h b/src/server/i18n.h
index e1cbcc68..79005b72 100644
--- a/src/server/i18n.h
+++ b/src/server/i18n.h
@@ -111,6 +111,12 @@ private: // data
   const Parameters  params;
 };
 
+inline ParameterizedMessage nonParameterizedMessage(const std::string& msgId)
+{
+  const ParameterizedMessage::Parameters noParams;
+  return ParameterizedMessage(msgId, noParams);
+}
+
 struct LangPreference
 {
   const std::string lang;
diff --git a/src/server/internalServer.cpp b/src/server/internalServer.cpp
index cdfa4deb..6f98b409 100644
--- a/src/server/internalServer.cpp
+++ b/src/server/internalServer.cpp
@@ -190,12 +190,6 @@ ParameterizedMessage tooManyBooksMsg(size_t nbBooks, size_t limit)
   );
 }
 
-ParameterizedMessage nonParameterizedMessage(const std::string& msgId)
-{
-  const ParameterizedMessage::Parameters noParams;
-  return ParameterizedMessage(msgId, noParams);
-}
-
 struct Error : public std::runtime_error {
   explicit Error(const ParameterizedMessage& message)
     : std::runtime_error("Error while handling request"),
@@ -650,11 +644,11 @@ std::unique_ptr<Response> InternalServer::handle_request(const RequestContext& r
   } catch (std::exception& e) {
     fprintf(stderr, "===== Unhandled error : %s\n", e.what());
     return HTTP500Response(*this, request)
-         + e.what();
+         + ParameterizedMessage("non-translated-text", {{"MSG", e.what()}});
   } catch (...) {
     fprintf(stderr, "===== Unhandled unknown error\n");
     return HTTP500Response(*this, request)
-         + "Unknown error";
+         + nonParameterizedMessage("unknown-error");
   }
 }
 
diff --git a/src/server/response.cpp b/src/server/response.cpp
index f03ee5fc..36a2da24 100644
--- a/src/server/response.cpp
+++ b/src/server/response.cpp
@@ -205,15 +205,11 @@ UrlNotFoundResponse::UrlNotFoundResponse(const InternalServer& server,
   *this += ParameterizedMessage("url-not-found", {{"url", requestUrl}});
 }
 
-HTTPErrorResponse& HTTPErrorResponse::operator+(const std::string& msg)
-{
-  m_data["details"].push_back({"p", msg});
-  return *this;
-}
-
 HTTPErrorResponse& HTTPErrorResponse::operator+(const ParameterizedMessage& details)
 {
-  return *this + details.getText(m_request.get_user_language());
+  const std::string msg = details.getText(m_request.get_user_language());
+  m_data["details"].push_back({"p", msg});
+  return *this;
 }
 
 HTTPErrorResponse& HTTPErrorResponse::operator+=(const ParameterizedMessage& details)
@@ -251,8 +247,7 @@ HTTP500Response::HTTP500Response(const InternalServer& server,
                       "500-page-title",
                       "500-page-heading")
 {
-  // operator+() is a state-modifying operator (akin to operator+=)
-  *this + "An internal server error occured. We are sorry about that :/";
+  *this += nonParameterizedMessage("500-page-text");
 }
 
 std::unique_ptr<ContentResponse> HTTP500Response::generateResponseObject() const
diff --git a/src/server/response.h b/src/server/response.h
index 32702de4..46ed4775 100644
--- a/src/server/response.h
+++ b/src/server/response.h
@@ -169,7 +169,6 @@ struct HTTPErrorResponse : ContentResponseBlueprint
                     const std::string& headingMsgId,
                     const std::string& cssUrl = "");
 
-  HTTPErrorResponse& operator+(const std::string& msg);
   HTTPErrorResponse& operator+(const ParameterizedMessage& errorDetails);
   HTTPErrorResponse& operator+=(const ParameterizedMessage& errorDetails);
 };
diff --git a/static/skin/i18n/en.json b/static/skin/i18n/en.json
index 97a71d55..8c4a6b0b 100644
--- a/static/skin/i18n/en.json
+++ b/static/skin/i18n/en.json
@@ -22,6 +22,7 @@
 	, "404-page-heading" : "Not Found"
 	, "500-page-title" : "Internal Server Error"
 	, "500-page-heading" : "Internal Server Error"
+	, "500-page-text": "An internal server error occured. We are sorry about that :/"
 	, "fulltext-search-unavailable" : "Fulltext search unavailable"
 	, "no-search-results": "The fulltext search engine is not available for this content."
 	, "library-button-text": "Go to welcome page"
@@ -52,4 +53,6 @@
 	, "download-links-heading": "Download links for <b><i>{{BOOK_TITLE}}</i></b>"
 	, "download-links-title": "Download book"
 	, "preview-book": "Preview"
+	, "non-translated-text": "{{MSG}}"
+	, "unknown-error": "Unknown error"
 }
diff --git a/static/skin/i18n/qqq.json b/static/skin/i18n/qqq.json
index d647a75e..4fa2ef94 100644
--- a/static/skin/i18n/qqq.json
+++ b/static/skin/i18n/qqq.json
@@ -25,6 +25,7 @@
 	"404-page-heading": "Heading of the 404 error page",
 	"500-page-title": "Title of the 500 error page",
 	"500-page-heading": "Heading of the 500 error page",
+	"500-page-text": "Text of the 500 error page",
 	"fulltext-search-unavailable": "Title of the error page returned when search is attempted in a book without fulltext search database",
 	"no-search-results": "Text of the error page returned when search is attempted in a book without fulltext search database",
 	"library-button-text": "Tooltip of the button leading to the welcome page",
@@ -53,5 +54,7 @@
 	"welcome-to-kiwix-server": "Title shown in browser's title bar/page tab",
 	"download-links-heading": "Heading for no-js download page",
 	"download-links-title": "Title for no-js download page",
-	"preview-book": "Tooltip of book-tile leading to the book"
+	"preview-book": "Tooltip of book-tile leading to the book",
+	"non-translated-text": "Used to display text that is generated at runtime and cannot be translated. Nothing to translate about this one.",
+	"unknown-error": "Unknown error"
 }