Add graphs and retention metrics to admin dashboard (#16829)

app/lib/admin/metrics/dimension/languages_dimension.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class Admin::Metrics::Dimension::LanguagesDimension < Admin::Metrics::Dimension::BaseDimension
+  def key
+    'languages'
+  end
+
+  def data
+    sql = <<-SQL.squish
+      SELECT locale, count(*) AS value
+      FROM users
+      WHERE current_sign_in_at BETWEEN $1 AND $2
+        AND locale IS NOT NULL
+      GROUP BY locale
+      ORDER BY count(*) DESC
+      LIMIT $3
+    SQL
+
+    rows = ActiveRecord::Base.connection.select_all(sql, nil, [[nil, @start_at], [nil, @end_at], [nil, @limit]])
+
+    rows.map { |row| { key: row['locale'], human_key: SettingsHelper::HUMAN_LOCALES[row['locale'].to_sym], value: row['value'].to_s } }
+  end
+end