Reproduction

frontend/src/metabase/admin/settings/components/widgets/FormattingWidget.tsx

@@ -158,7 +158,7 @@ export function FormattingWidget() {
                 { label: "100 000,00", value: ", " },
                 { label: "100.000,00", value: ",." },
                 { label: "100000.00", value: "." },
-                { label: "100'000.00", value: ".'" },
+                { label: "100’000.00", value: ".’" },
               ]}
               onChange={(newValue) =>
                 handleChange({

src/metabase/appearance/settings.clj

@@ -341,6 +341,23 @@ See [fonts](../configuring-metabase/fonts.md).")
   :audit      :getter
   :feature    :whitelabel)
 
+(def avaialable-number-separators
+  "Number separators that are available to use in uploads csv parsing. Values match what is implemented
+  `metabase.upload.types` namespace, specifically e.g. [[metabase.upload.types/int-regex]]."
+  #{"."
+    ".,"
+    ",."
+    ", "
+    ".’"})
+
+(defn- validate-custom-formatting!
+  [new-value]
+  (let [separators (some-> new-value :type/Number :number_separators)]
+    (when-not (avaialable-number-separators separators)
+      (throw (ex-info (tru "Invalid number separators.")
+                      {:separators separators
+                       :available-separators avaialable-number-separators})))))
+
 (defsetting custom-formatting
   (deferred-tru "Object keyed by type, containing formatting settings")
   :encryption :no
@@ -348,7 +365,10 @@ See [fonts](../configuring-metabase/fonts.md).")
   :export?    true
   :default    {}
   :visibility :public
-  :audit      :getter)
+  :audit      :getter
+  :setter     (fn [new-value]
+                (validate-custom-formatting! new-value)
+                (setting/set-value-of-type! :json :custom-formatting new-value)))
 
 (defsetting show-homepage-data
   (deferred-tru

test/metabase/appearance/settings_test.clj

@@ -178,3 +178,15 @@
       (is (thrown-with-msg? clojure.lang.ExceptionInfo
                             #"Loading message set to an unsupported value"
                             (appearance.settings/loading-message! :unsupported-value))))))
+
+(deftest custom-formatting-number-separators-test
+  (testing "Only valid values can be set as number separators (#61854)"
+    (let [violating-separators ".'"]
+      (is (thrown-with-msg?
+           Exception #"Invalid number separators."
+           (appearance.settings/custom-formatting!
+            #:type{:Temporal {:date_style "MMMM D, YYYY"
+                              :time_style "h:mm A"
+                              :date_abbreviate false}
+                   :Number {:number_separators violating-separators}
+                   :Currency {:currency "USD" :currency_style "symbol"}}))))))