docs(design): add UX writing guide for user-facing strings

Adds a new writing.rst page to developer_manual/design/ covering tone,
message brevity, the "successfully" anti-pattern, button label conventions,
and variable/placeholder gotchas for translators.

Linked from design/index.rst toctree.

Relates to #13884

Signed-off-by: John Molakvoæ (skjnldsv) <skjnldsv@protonmail.com>

Author: skjnldsv

developer_manual/design/index.rst

@@ -6,6 +6,7 @@ Interface & interaction design
     :maxdepth: 2
 
     introduction
+    writing
     foundations
     layout
     layoutcomponents

developer_manual/design/writing.rst

@@ -0,0 +1,117 @@
+.. _ux-writing:
+
+=============
+Writing guide
+=============
+
+Consistent, concise wording makes Nextcloud easier to use and easier to translate.
+Follow these rules when writing any user-facing string: notifications, dialog messages, button labels, error text, tooltips.
+
+General rules
+-------------
+
+**Keep messages short.** One idea per sentence. Cut every word that does not add meaning.
+
+**Use sentence case.** Capitalize only the first word and proper nouns.
+
+.. list-table::
+   :header-rows: 1
+   :widths: 50 50
+
+   * - Avoid
+     - Prefer
+   * - Settings Saved Successfully
+     - Settings saved
+   * - An Error Has Occurred While Loading The File
+     - Could not load the file
+   * - Your changes have been successfully applied to the system
+     - Changes saved
+
+**Drop "successfully".** If an action completed, the result speaks for itself.
+State what happened, not that it happened without error.
+
+.. list-table::
+   :header-rows: 1
+   :widths: 50 50
+
+   * - Avoid
+     - Prefer
+   * - Settings saved successfully
+     - Settings saved
+   * - User created successfully
+     - User created
+   * - File uploaded successfully
+     - File uploaded
+
+**Be specific in error messages.** Tell users what went wrong and, where possible, what to do next.
+
+.. list-table::
+   :header-rows: 1
+   :widths: 50 50
+
+   * - Avoid
+     - Prefer
+   * - An error occurred
+     - Could not save settings. Check your connection and try again.
+   * - Invalid input
+     - Password must be at least 8 characters
+
+**Avoid technical jargon** unless the audience is explicitly technical (e.g. a developer-facing admin panel).
+Use plain language for anything end users see.
+
+Tone
+----
+
+- **Friendly, not chatty.** Write like a knowledgeable colleague, not a marketing brochure.
+- **Direct, not bossy.** Prefer statements over commands in status messages.
+- **Neutral, not emotional.** Avoid exclamation marks in status or error text.
+
+.. list-table::
+   :header-rows: 1
+   :widths: 50 50
+
+   * - Avoid
+     - Prefer
+   * - Great! Your profile has been updated!
+     - Profile updated
+   * - Oops! Something went wrong!
+     - Could not complete the request
+
+Button and action labels
+------------------------
+
+Use **verb + noun** for buttons that trigger an action. The noun can be omitted when context makes it obvious.
+
+.. list-table::
+   :header-rows: 1
+   :widths: 50 50
+
+   * - Avoid
+     - Prefer
+   * - OK
+     - Save settings
+   * - Submit
+     - Create account
+   * - Yes
+     - Delete file
+
+For destructive actions use the specific verb so users know exactly what will happen: **Delete**, **Remove**, **Revoke** — not **Confirm** or **OK**.
+
+Placeholders and variables
+--------------------------
+
+When a string contains a variable (file name, user name, count), keep the surrounding text short and natural.
+Make sure the string still makes sense in all languages — word order differs across languages, so avoid splitting a sentence across two separate strings.
+
+.. code-block:: php
+
+   // Good — full sentence in one string, variable embedded
+   $l->t('Shared with %s', [$userName]);
+
+   // Avoid — concatenation breaks translation
+   $l->t('Shared with ') . $userName;
+
+Translatable strings
+--------------------
+
+For implementation details on marking strings as translatable in PHP, JavaScript, and Vue, see :doc:`../basics/translations`.