_form.html.erb

<div class="input short">
  <%= data_f.label :number_format, _("Style de formatage des nombres") %>
  <%= data_f.select :number_format, NumberFormat.options_for_select %>
</div>

<div class="input short">
  <%= data_f.label :date_format, _("Style de formatage des dates") %>
  <%= data_f.select :date_format, DateFormat.options_for_select %>
</div>

app.rb

NumberFormat.format(current_user.number_format, @invoice.amount)
DateFormat.format(current_user.date_format, @invoice.issue_date)

date_format.rb

module DateFormat
  CODES = ['DMY', 'MDY']
  
  DATE_FORMATTING_STYLES = [
    ['31/12/2001', '%d/%m/%Y'],
    ['12/31/2001', '%m/%d/%Y']
  ]
  
  def self.format(code, date)
    if code == 'DMY'
      date.strftime('%d/%m/%Y')
    elsif code == 'MDY'
      date.strftime('%m/%d/%Y')
    else
      raise "Unknown date format code #{code}"
    end
  end
  
  def self.options_for_select
    [
      ['31/12/2001', 'DMY'],
      ['12/31/2001', 'MDY']
    ]
  end
  
  def self.for_jQuery(code)
    if code == 'DMY'
      'dd/mm/yy'
    elsif code == 'MDY'
      'mm/dd/yy'
    else
      raise "Unknown date format code #{code}"
    end
  end
end

model.rb

class User
  include Mongoid::Document

  field :locale,        type: String
  field :number_format, type: String
  field :date_format,   type: String
end

number_format.rb

module NumberFormat
  
  CODES = ['COMMA', 'DOT', 'SWISS']
  
  def self.format(code, number)
    if code == 'COMMA'
      separator = ','
      delimiter = ' '
    elsif code == 'DOT'
      separator = '.'
      delimiter = ','
    elsif code == 'SWISS'
      separator = '.'
      delimiter = "'"
    else
      raise "Unknown number format : #{code}"
    end
    
    number_with_precision(number, {
      :separator                 => separator,
      :delimiter                 => delimiter,
      :precision                 => 2,
      :strip_insignificant_zeros => false
    })#.html_safe
  end
  
  def self.options_for_select
    [
      ['10 926,42', 'COMMA'],
      ['10,926.42', 'DOT'],
      ["10'926,42", 'SWISS']
    ]
  end
  
  def self.number_with_precision(number, options)
    ActionController::Base.helpers.number_with_precision(number, options)
  end
end