How globalize gem decides which content goes to translated table? - ruby-on-rails

Question

In my rails 4.2 app I have set it as the default locale by setting it in config/application.rb:
config.i18n.default_locale = :it
I have a simple "product" model
class Product < ActiveRecord::Base
translates :name, :description
end
My need is when I18n.locale = :it the content should be written in "products" table while for all others locales the content shoud go in the "product_translations" table.
Currently what happens is the following:
if
config.i18n.default_locale = :en
content is written to the "products" table, for all different locales the content goes to the "product_translations" table.
How can I change this?
EDIT
Using the console to test globalize behaviour I found that maybe I did not understand how globalize should work.
I was expecting that the "products" table is filled with default_locale (in my case :it) and the "product_translations" table is filled with other locales (:en, :fr, :de and so on).
Instead I see that whichever the locale is, fields that are indicated as
translates :name, :description
are always written in "product_translations" and "product" table only contains those fields that are not translated (in my case uom (unit of measure).
This is the output of the console after saving a new product with :en as locale.
[18] pry(main)> en_p=Product.create(:name=>"butter",
:description => "82% min fat butter",
:uom => "kg") (0.3ms)
BEGIN
SQL (0.7ms)
INSERT INTO "products" ("uom", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"
[
["uom", "kg"],
["created_at", "2015-07-14 05:49:09.097092"],
["updated_at", "2015-07-14 05:49:09.097092"]
]
SQL (0.7ms)
INSERT INTO "product_translations" ("locale",
"name",
"description",
"product_id",
"created_at",
"updated_at")
VALUES ($1, $2, $3, $4, $5, $6) RETURNING "id"
[
["locale", "en"],
["name", "butter"],
["description", "82% min fat butter"],
["product_id", 5],
["created_at", "2015-07-14 05:49:09.116683"],
["updated_at", "2015-07-14 05:49:09.116683"]
]
(15.9ms)
COMMIT
=> #<Product:0xb63d3568
id: 5,
name: "butter",
description: "82% min fat butter",
uom: "kg",
created_at: Tue, 14 Jul 2015 05:49:09 UTC +00:00,
updated_at: Tue, 14 Jul 2015 05:49:09 UTC +00:00>
[19] pry(main)> I18n.locale=:it
=> :it
[20] pry(main)> it_p=Product.create(:name=>"olio di oliva EVO",
:description => "Olio di oliva extravergine spremuto a freddo",
:uom => "kg") (0.4ms)
BEGIN
SQL (0.5ms)
INSERT INTO "products" ("uom", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"
[
["uom", "kg"],
["created_at", "2015-07-14 05:51:34.772755"],
["updated_at", "2015-07-14 05:51:34.772755"]
]
SQL (0.8ms)
INSERT INTO "product_translations" ("locale",
"name",
"description",
"product_id",
"created_at",
"updated_at")
VALUES ($1, $2, $3, $4, $5, $6) RETURNING "id"
[
["locale", "it"],
["name", "olio di oliva extravergine"],
["description", "Olio di oliva extravergine ottenuto unicamente per spremitura"],
["product_id", 6],
["created_at", "2015-07-14 05:51:34.779220"],
["updated_at", "2015-07-14 05:51:34.779220"]
]
(16.1ms)
COMMIT
=> #<Product:0xb6315ce8
id: 6,
name: "olio di oliva extravergine",
description: "Olio di oliva extravergine ottenuto unicamente per spremitura",
uom: "kg",
created_at: Tue, 14 Jul 2015 05:51:34 UTC +00:00,
updated_at: Tue, 14 Jul 2015 05:51:34 UTC +00:00>
Is this the default behaviour?
Do I need to remove translatable fields from the original table?