[mob[ Added doc for mobile translations + added 2 new source string… (#4040)

This is my first contribution and I wanted to start with something
'simple'. My goal is to further contribute to the translation of Ente
photos. I am trying to understand that process which is not yet entirely
clear to me, especially when it comes to which files must be (or do not
need to be) generated.

## Description
As a first (small) step, I have:
- created 2 new source strings ('collaboratorsSuccessfullyAdded' and
'viewersSuccessfullyAdded') that replace hardcoded text in the
"add_a_participant" page.
- Created draft of a documentation for translations on mobile app (based
on the doc for web app)

## Tests

Tested on my Pixel 6a

Built with :
- Flutter 3.24.3
- JDK 17.0.2
- Gradle 7.2

## Questions
- I have manually updated the `l10n.dart` file (using existing similar
strings ) but I guess it should be automatically generated from the
`intl_en.arb` file, correct? If so, is it something anyone can do and
can we add the step in the doc?
- Should the other languages be updated as well in that commit, or will
they only be updated via the Github Crowdin action ? Looking at the
existing PR, it seems like they might need to be updated as well in the
commit.

mobile/docs/translations.md

@@ -0,0 +1,49 @@
+# Translations
+
+We use Crowdin for translations, and the `intl` package to load these at
+runtime.
+
+Within our project we have the _source_ strings - these are the key value pairs
+in the `lib/l10n/intl_en.arb` file.
+
+Volunteers can add a new _translation_ in their language corresponding to each
+such source key-value to our
+[Crowdin project](https://crowdin.com/project/ente-photos-app).
+
+When a new source string is added, we run a [GitHub workflow](../../.github/workflows/mobile-crowdin-push.yml)
+that
+
+-   Uploads sources to Crowdin - So any new key value pair we add in the source
+    `intl_en.arb` becomes available to translators to translate.
+
+Every monday, we run a [GitHub workflow](../../.github/workflows/mobile-crowdin-sync.yml)
+that 
+
+-   Downloads translations from Crowdin - So any new translations that
+    translators have made on the Crowdin dashboard (for existing sources) will
+    be added to the corresponding `intl_XX.arb`.
+
+The workflow also uploads existing translations and also downloads new sources
+from Crowdin, but these two should be no-ops.
+
+## Adding a new string
+
+-   Add a new entry in `lib/l10n/intl_en.arb` (the
+    **source `intl_en.arb`**).
+-   Use the new key in code with the `S` class
+    (`import "package:photos/generated/l10n.dart"`).
+-   During the next sync, the workflow will upload this source item to Crowdin's
+    dashboard, allowing translators to translate it.
+
+## Updating an existing string
+
+-   Update the existing value for the key in the source `intl_en.arb`.
+-   During the next sync, the workflow will clear out all the existing
+    translations so that they can be translated afresh.
+
+## Deleting an existing string
+
+-   Remove the key value pair from the source `intl_en.arb`.
+-   During the next sync, the workflow will delete that source item from all
+    existing translations (both in the Crowdin project and also from the
+    other `intl_XX.arb` files in the repository).

mobile/lib/generated/l10n.dart

@@ -9945,6 +9945,32 @@ class S {
       args: [],
     );
   }
+
+  /// `{count, plural, =0 {Added 0 viewer} =1 {Added 1 viewer} other {Added {count} viewers}}`
+  String viewersSuccessfullyAdded(int count) {
+    return Intl.plural(
+      count,
+      zero: 'Added 0 viewer',
+      one: 'Added 1 viewer',
+      other: 'Added $count viewers',
+      name: 'viewersSuccessfullyAdded',
+      desc: 'Number of viewers that were successfully added to an album.',
+      args: [count],
+    );
+  }
+
+  /// `{count, plural, =0 {Added 0 collaborator} =1 {Added 1 collaborator} other {Added {count} collaborators}}`
+  String collaboratorsSuccessfullyAdded(int count) {
+    return Intl.plural(
+      count,
+      zero: 'Added 0 collaborator',
+      one: 'Added 1 collaborator',
+      other: 'Added $count collaborators',
+      name: 'collaboratorsSuccessfullyAdded',
+      desc: 'Number of collaborators that were successfully added to an album.',
+      args: [count],
+    );
+  }
 }
 
 class AppLocalizationDelegate extends LocalizationsDelegate<S> {

mobile/lib/l10n/intl_en.arb

@@ -1363,5 +1363,25 @@
   "checkingModels": "Checking models...",
   "enableMachineLearningBanner": "Enable machine learning for magic search and face recognition",
   "searchDiscoverEmptySection": "Images will be shown here once processing is complete",
-  "searchPersonsEmptySection": "People will be shown here once processing is complete"
+  "searchPersonsEmptySection": "People will be shown here once processing is complete",
+  "viewersSuccessfullyAdded": "{count, plural, =0 {Added 0 viewer} =1 {Added 1 viewer} other {Added {count} viewers}}",
+  "@viewersSuccessfullyAdded": {
+    "placeholders": {
+      "count": {
+        "type": "int",
+        "example": "2"
+      }
+    },
+    "description": "Number of viewers that were successfully added to an album."
+  },
+  "collaboratorsSuccessfullyAdded": "{count, plural, =0 {Added 0 collaborator} =1 {Added 1 collaborator} other {Added {count} collaborators}}",
+  "@collaboratorsSuccessfullyAdded": {
+    "placeholders": {
+      "count": {
+        "type": "int",
+        "example": "2"
+      }
+    },
+    "description": "Number of collaborators that were successfully added to an album."
+  }
 }

mobile/lib/ui/sharing/add_participant_page.dart

@@ -239,7 +239,7 @@ class _AddParticipantPage extends State<AddParticipantPage> {
                           results.where((e) => e).length;
                       showToast(
                         context,
-                        "Added $noOfSuccessfullAdds ${widget.isAddingViewer ? "viewers" : "collaborators"}",
+                        widget.isAddingViewer ? S.of(context).viewersSuccessfullyAdded(noOfSuccessfullAdds) : S.of(context).collaboratorsSuccessfullyAdded(noOfSuccessfullAdds),
                       );
 
                       if (!results.any((e) => e == false) && mounted) {