Fix follow recommendation moderation page default language when using regional variant (#32580)

app/models/follow_recommendation_filter.rb

@@ -11,7 +11,7 @@ class FollowRecommendationFilter
   attr_reader :params, :language
 
   def initialize(params)
-    @language = params.delete('language') || I18n.locale
+    @language = usable_language(params.delete('language') || I18n.locale)
     @params   = params
   end
 
@@ -22,4 +22,15 @@ class FollowRecommendationFilter
       Account.includes(:account_stat).joins(:follow_recommendation).merge(FollowRecommendation.localized(@language).order(rank: :desc))
     end
   end
+
+  private
+
+  def usable_language(locale)
+    return locale if Trends.available_locales.include?(locale)
+
+    locale = locale.to_s.split(/[_-]/).first
+    return locale if Trends.available_locales.include?(locale)
+
+    nil
+  end
 end