Use plan.owner.language for plan.language

Prior to this change, `LocaleService.default_locale` was always assigned to `json.language`.
- Now we use `plan.owner&.language&.abbreviation` (or `LocaleService.default_locale` as a fallback)

Author: aaronskiba

app/presenters/api/v2/language_presenter.rb

@@ -75,6 +75,7 @@ class << self
         # Convert the incoming 2 (e.g. en - ISO 639-1) or 2+region (e.g. en-UK)
         # into the 3 character code (e.g. eng - ISO 639-2)
         def three_char_code(lang:)
+          lang ||= LocaleService.default_locale
           two_char_code = lang.to_s.split('-').first
           LANGUAGE_MAP[two_char_code.to_sym]
         end

app/views/api/v2/plans/_show.json.jbuilder

@@ -14,7 +14,7 @@ presenter = Api::V2::PlanPresenter.new(plan: plan, complete: @complete)
 json.title plan.title
 json.description plan.description
 json.language Api::V2::LanguagePresenter.three_char_code(
-  lang: LocaleService.default_locale
+  lang: plan.owner&.language&.abbreviation
 )
 json.created plan.created_at.to_formatted_s(:iso8601)
 json.modified plan.updated_at.to_formatted_s(:iso8601)