Get search dictionary based on I18n.default_locale (merge pull request #3856)

Implementation tries to be open for further extensions, such as deciding on
search dictionary based on configuration option or by locale set for
given user.

lib/search_dictionary_selector.rb

@@ -0,0 +1,42 @@
+module SearchDictionarySelector
+  SQL_QUERY = "SELECT cfgname FROM pg_ts_config".freeze
+  I18N_TO_DICTIONARY = {
+    en: "english",
+    de: "german",
+    fi: "finnish",
+    fr: "french",
+    dk: "danish",
+    nl: "dutch",
+    hu: "hungarian",
+    it: "italian",
+    nn: "norwegian",
+    nb: "norwegian",
+    pt: "portuguese",
+    ro: "romanian",
+    ru: "russian",
+    es: "spanish",
+    sv: "swedish",
+    tr: "turkish"
+  }.freeze
+
+  class << self
+    def call
+      find_from_i18n_default
+    end
+
+    private
+
+      def find_from_i18n_default
+        key_to_lookup = I18n.default_locale.to_s.split("-").first.to_sym
+
+        dictionary = I18N_TO_DICTIONARY[key_to_lookup]
+        dictionary ||= "simple"
+        available_dictionaries.include?(dictionary) ? dictionary : available_dictionaries.first
+      end
+
+      def available_dictionaries
+        result = ActiveRecord::Base.connection.execute(SQL_QUERY)
+        result.to_a.map { |row| row["cfgname"] }
+      end
+  end
+end