fix(date): show correct format based on browser locale

app/components/editable_champ/champ_label_content_component/champ_label_content_component.html.haml

@@ -18,7 +18,7 @@
     = t('.check_content_rebased')
 
 - if hint?
-  %span.fr-hint-text= hint
+  %span.fr-hint-text{ data: { controller: 'date-input-hint' } }= hint
 
 - if @champ.description.present?
   %span.fr-hint-text{ id: @champ.describedby_id }= render SimpleFormatComponent.new(@champ.description, allow_a: true)

app/components/editable_champ/date_component/date_component.html.haml

@@ -3,4 +3,4 @@
   aria: { describedby: @champ.describedby_id },
   value: @champ.value,
   required: @champ.required?,
-  placeholder: 'aaaa-mm-jj', class: "width-33-desktop")
+  class: "width-33-desktop")

app/javascript/controllers/date_input_hint_controller.ts

@@ -0,0 +1,49 @@
+import { ApplicationController } from './application_controller';
+
+const DATE_PLACEHOLDER_REGEXP = /[jJmMdD]{2}\/[jJmMdD]{2}\/[aAyY]{4}/;
+const DATE_EXAMPLE_REGEXP = /\d{1,2}\/\d{1,2}\/\d{4}/;
+const PARTS = {
+  fr: {
+    '15': 'JJ',
+    '10': 'MM',
+    '2022': 'AAAA'
+  },
+  en: {
+    '15': 'dd',
+    '10': 'mm',
+    '2022': 'yyyy'
+  }
+};
+
+export class DateInputHintController extends ApplicationController {
+  connect() {
+    this.fixDateFormat(this.element);
+  }
+
+  private fixDateFormat(input: Element) {
+    const text = input.textContent ?? '';
+    const match = text.match(DATE_PLACEHOLDER_REGEXP);
+
+    if (match) {
+      const [placeholder, example] = this.translatePlaceholder();
+      // This component behaviour can be had to test and debug. We keep a (debug) log here to help.
+      console.debug(`Replace ${match[0]} with ${placeholder} and ${example}`);
+      input.textContent = text
+        .replace(DATE_PLACEHOLDER_REGEXP, placeholder)
+        .replace(DATE_EXAMPLE_REGEXP, example);
+    }
+  }
+
+  private translatePlaceholder() {
+    const locale = document.documentElement.lang as 'fr' | 'en';
+    const parts = PARTS[locale];
+    const example = new Date(2022, 9, 15).toLocaleDateString();
+    return [
+      Object.entries(parts).reduce(
+        (text, [part, str]) => text.replace(part, str),
+        example
+      ),
+      example
+    ];
+  }
+}