feat: Add language parameter to add_parcel service

[robbybarnes]

Summary

Adds support for the language parameter when adding new parcels via the add_parcel service. This allows users to specify their preferred language for tracking updates.

Motivation

The ParcelApp API supports a language parameter in the /external/add-delivery/ endpoint (see API docs), but this integration wasn't exposing it. Users in non-English speaking regions may want tracking updates in their native language.

Changes

• Added LANGUAGE and DEFAULT_LANGUAGE constants to const.py
• Updated ADD_PARCEL_SCHEMA in services.py to accept optional language parameter
• Modified async_add_parcel to include language in API payload
• Added field description in services.yaml
• Added translation strings in translations/en.json

Usage

service: parcelapp.add_parcel
data:
  parcel_name: "My Package"
  tracking_number: "1234567890"
  courier: "fedex"
  language: "de"  # Optional, defaults to "en"

Testing

• All existing tests pass (11/11)
• Manually verified schema validation accepts the new parameter

[PineappleEmperor]

Hey @robbybarnes! Thanks very much for all the PRs, I'm working my way through them and will leave some review comments as I go.

This one seems to be fairly straight-forward, though I haven't been able to properly test the language feature; from what I can see it's passed correctly as per the API docs at least!

The only comment I have on this one is I feel the UX is better if the notificaiton option in the service is last, after the language option.

[PineappleEmperor]

If you can rearrange the services.yaml I think it's good to go! 🚀

[jmdevita]

LGTM if @PineappleEmperor agrees!

[PineappleEmperor]

Happy to go ahead :) Do we want to hold off releasing a new version until a few more of these are done and merged?

[jmdevita]

I'm thinking we package a few of these together so that they all merge into a new release, I'll be taking a look in the next few days as well!