Ruby array to sentence string

It's very simple:

irb> ["one","two","three"].to_sentence 
=> "one, two and three"

you can use different options to humanize output:

irb> ["one","two","three"].to_sentence(words_connector: " not equal ")
=> "one not equal two and three"

irb> ["to be", "not to be"].to_sentence(two_words_connector: " or ")
=> "to be or not to be"

But remember about your default locale in Rails!

# application.rb 
config.i18n.default_locale = :ru

irb> ["one","two","three"].to_sentence
=> "one, two и three"

To read:
Array#to_sentence (apidoc)