hi,
I've been asked to design a multiple language application for an
architectural firm and I need advice with which is the best approach
to it with Rails.
Basically all the tables have some common fields that doesn't need to
be translated and some others that need translation.
At first it seems the application will be quite changing since not all
the targets are clearly defined so, ease of change will make a good
point.

best regards,
indi

Just right now I'm solving definitely this kind of task.

I choose an approach where each content having to be translated, is
String or Text.
Then I created two STI+polymorphic tables (models): MetaText and
MetaString. Each of them has columns "value" and "locale" to specify
language.

class MetaString < ActiveRecord::Base
  attr_accessible :value, :locale
  belongs_to :resource, :polymorphic => true
end

class MetaText < ActiveRecord::Base
  attr_accessible :value, :locale
  belongs_to :resource, :polymorphic => true
end

For each model I've created STI models for whatever string/text content
to be translated. STI is needed to specify model behavior, such as
validation or etc...

Example:

class ArticleTitle < MetaString
   validates_lenght_of :value, :maximum => 200
end

class ArticleContent < MetaText
   validates_lenght_of :value, :minimum => 200
end

class Article < ActiveRecord::Base
  has_many :titles, :class_name => "ArticleTitle", :as => :resource,
:dependent => :delete_all
  has_many :contents, :class_name => "ArticleTitle", :as => :resource,
:dependent => :delete_all

  def title(locale=nil)
    items = titles.to_a
    items.find {|i| i.locale == locale || I18n.locale} || items.first
  end

  def content(locale=nil)
    items = contents.to_a
    items.find {|i| i.locale == locale || I18n.locale} || items.first
  end

end

I18n.locale - application-level locale accessor.
I choose an approach (subdomain = locale):
- en.mysite.com - locale is 'en'
- ru.mysite.com - locale is 'ru' etc

http://railscasts.com/episodes/221-subdomains-in-rails-3