diff --git a/kagi-test.el b/kagi-test.el
index f1d667d..a950e9a 100644
--- a/kagi-test.el
+++ b/kagi-test.el
@@ -238,22 +238,63 @@ https://www.example.com"
         (expect (call-interactively #'kagi-proofread) :to-throw))))
 
   (describe "Summarizer"
-    :var ((just-enough-input nil)
-          (just-too-little-input nil))
+    :var ((just-enough-text-input nil)
+          (just-too-little-text-input nil)
+          (dummy-https-url "https://www.example.com")
+          (dummy-http-url "http://www.example.com")
+          (dummp-ftp-url "ftp://example.com"))
     (before-all
-      (dotimes (_ 50) (push "a" just-enough-input))
-      (setq just-too-little-input (string-join (cdr just-enough-input) " "))
-      (setq just-enough-input (string-join just-enough-input " ")))
+      (dotimes (_ 50) (push "a" just-enough-text-input))
+      (setq just-too-little-text-input (string-join (cdr just-enough-text-input) " "))
+      (setq just-enough-text-input (string-join just-enough-text-input " ")))
+    (before-each
+      (spy-on #'kagi--call-summarizer :and-call-through))
     (describe "kagi-summarize"
-      (it "returns a summary on minimal input"
-        (expect (kagi-summarize just-enough-input) :to-equal dummy-output))
-      (it "throws on just too little output"
-        (expect (kagi-summarize just-too-little-input) :to-throw))
-      (it "throws an error on too little input"
+      :var ((kagi-summarizer-default-language))
+      (before-each
+        (setq kagi-summarizer-default-language "XY"))
+      (it "returns a summary on minimal text input"
+        (expect (kagi-summarize just-enough-text-input) :to-equal dummy-output))
+      (it "makes exactly one API call"
+        (kagi-summarize just-enough-text-input)
+        (expect #'kagi--call-api :to-have-been-called-times 1))
+      (it "throws on just too little text output"
+        (expect (kagi-summarize just-too-little-text-input) :to-throw))
+      (it "throws an error on too little text input"
         (expect (kagi-summarize "foo") :to-throw))
       (it "throws an error on empty input"
         (expect (kagi-summarize "") :to-throw))
       (it "throws an error on missing input"
-        (expect (kagi-summarize nil) :to-throw)))))
+        (expect (kagi-summarize nil) :to-throw))
+      (it "returns a summary for a valid language code"
+        (expect (kagi-summarize just-enough-text-input "NL" :to-equal dummy-output))
+        (let ((args (car (spy-calls-args-for #'kagi--call-summarizer 0))))
+          (expect (map-elt args "target_language") :to-equal "NL")))
+      (it "returns a summary for a valid language code with wrong capitalization"
+        (expect (kagi-summarize just-enough-text-input "nL" :to-equal dummy-output))
+        (let ((args (car (spy-calls-args-for #'kagi--call-summarizer 0))))
+          (expect (map-elt args "target_language") :to-equal "NL")))
+      (it "returns a summary for a valid language name"
+        (expect (kagi-summarize just-enough-text-input "Dutch" :to-equal dummy-output))
+        (let ((args (car (spy-calls-args-for #'kagi--call-summarizer 0))))
+          (expect (map-elt args "target_language") :to-equal "NL")))
+      (it "returns a summary for a valid language name with different capitalization"
+        (expect (kagi-summarize just-enough-text-input "dUtch" :to-equal dummy-output))
+        (let ((args (car (spy-calls-args-for #'kagi--call-summarizer 0))))
+          (expect (map-elt args "target_language") :to-equal "NL")))
+      (it "falls back to the default language for invalid language codes"
+        (expect (kagi-summarize just-enough-text-input "VL") :to-equal dummy-output)
+        (let ((args (car (spy-calls-args-for #'kagi--call-summarizer 0))))
+          (expect (map-elt args "target_language") :to-equal "XY")))
+      (it "falls back to the default language for invalid language names"
+        (expect (kagi-summarize just-enough-text-input "Valyrian") :to-equal dummy-output)
+        (let ((args (car (spy-calls-args-for #'kagi--call-summarizer 0))))
+          (expect (map-elt args "target_language") :to-equal "XY")))
+      (it "returns a summary for an HTTPS URL"
+        (expect (kagi-summarize dummy-https-url) :to-equal dummy-output))
+      (it "returns a summary for an HTTP URL"
+        (expect (kagi-summarize dummy-http-url) :to-equal dummy-output))
+      (it "throws for an unsupported URL scheme"
+        (expect (kagi-summarize dummy-ftp-url) :to-throw)))))
 
 ;;; kagi-test.el ends here
diff --git a/kagi.el b/kagi.el
index 457acb2..2d73baf 100644
--- a/kagi.el
+++ b/kagi.el
@@ -513,6 +513,15 @@ no issues."
   "Non-nil if string S is a URL."
   (string-match-p (rx (seq bos "http" (? "s") "://" (+ (not space)) eos)) s))
 
+(defun kagi--summarizer-determine-language (hint)
+  "Determine the language for the summary given a language HINT."
+  (if hint
+      (or
+       (map-elt kagi--summarizer-languages (capitalize hint))
+       (and (seq-contains-p (map-values kagi--summarizer-languages) (upcase hint)) (upcase hint))
+       kagi-summarizer-default-language)
+    kagi-summarizer-default-language))
+
 (defun kagi-summarize (text-or-url &optional language engine format)
   "Return the summary of the given TEXT-OR-URL.
 
@@ -526,9 +535,7 @@ defined in `kagi--summarizer-engines'.
 FORMAT is the summary format, where `summary' returns a paragraph
 of text and `takeaway' returns a bullet list."
   (let* ((kagi-summarizer-default-language
-          (if (stringp language)
-              (upcase language)
-            kagi-summarizer-default-language))
+          (kagi--summarizer-determine-language language))
          (kagi-summarizer-engine
           (if (stringp engine)
               (downcase engine)