Send notification emails in the recipient's preferred language

[Copilot]

• Explore codebase to understand notification and language systems
• Modify _issue_watchers() to return email + language per recipient
• Add _set_language() helper to temporarily set language per recipient
• Update all notification methods to render per-recipient in their language
• Fix _set_language() return type to ?string and remove (string) cast so a null LANGUAGE is restored as null, not ""

Original prompt

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This section details on the original issue you should resolve

<issue_title>Notification emails are not translated</issue_title>
<issue_description>The notifications should use the user's language setting, followed by the global language setting to select translations for notification email components.
</issue_description>

<agent_instructions>Ensure notification emails are sent in the recipient user's language, if the user has a language manually set.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@Alanaktion Notification email language currently matches the sender's language, not the recipient's. We need to cleanly change the app language to match the user's preference when it's set. This isn't perfect, as many users will be using an English-language site, but let their browser determine the display language automatically when browsing, which we don't keep track of when that user isn't present. The best we can do with the current data is send emails following the user's manual setting if there is one, and hope the default on the site works if it's not set.

• Fixes Notification emails are not translated #80

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[Copilot]

Pull request overview

This PR fixes notification email localization by switching template rendering to run per-recipient using each recipient’s stored language preference, instead of using the sender/current session language.

Changes:

• Updated issue notification senders to render templates inside the per-recipient loop and temporarily switch LANGUAGE for each recipient.
• Extended _issue_watchers() to return email + language pairs and adjusted related SQL queries (including group membership lookup).
• Added _set_language(?string $language): string helper and applied it to non-issue notifications (user_reset, user_due_issues).

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[Copilot]

_set_language() always casts the previous LANGUAGE to a string and callers always restore via set("LANGUAGE", $previous). If LANGUAGE was previously unset/null, this forces it to "" on restore, which can change how locale fallback behaves. Consider tracking whether LANGUAGE originally existed and using clear("LANGUAGE") (or not restoring) when it didn’t.

[Alanaktion]

@copilot apply changes based on this feedback

If the original value was null, it should be null when changing back. Don't skip restoring.

[Copilot]

Fixed in 348ff36. Changed the return type of _set_language() from string to ?string and removed the (string) cast when reading the original value, so a null LANGUAGE is preserved and restored as null rather than "".