Prepare Drupal to be multilingual in the future I am building a Drupal website with only one language (Italian first) but there might be more languages in the future. This website will use also DrupalGap to create an app.
Editors don't need backend Italian interface.

So these are the scenarios I thought about:

Case one:


Install Drupal with English as default.
I will NOT enable Locale module.
All machine names (Content types, vocabularies, views etc..) will be in English.
Labels will in Italian.
Content and term will be created in Italian.
Views page title and views block title will be in Italian.


After few months the website may be translated in English so, the steps will be:


Enable locale.
Install Italian language.
Install Entity translation i18n etc...
Make entities translatable.
Bulk update all nodes and terms to be Italian as original language.
Revert all Italian labels into English and use string translation to translate it in Italian.


Case two:


Install Drupal with Italian as default
All machine names (Content types, vocabularies, views etc..) will be in English.
Labels will in Italian.
Content and term will be created in Italian.
Views page title and views block title will be in Italian.


After few months the website may be translated in English so, the steps will be:


Install Entity translation i18n etc...
Make entities translatable.
Bulk update all nodes and terms to be Italian as original language.
Translate strings from Italian to English.


In terms of extendability and usability, which approach should I use at the beginning?

Let me know if you have any question.

Thanks in advance.
I think the best approach is something similar to your first one.


Install Drupal in English as defaults 
Configure your environment for multilingual at the beginning.
Enable Italian Language
Set Italian to default language
When you create entities(Users, Content types, Etc..), configure everything at the beginning. Otherwise you might forget things later.
Enable multilingual support only for other pages excepting admin pages @ admin/config/regional/i18n/select (Under "ENABLE FOR SPECIFIC PAGES" set all pages except "admin/*")
Then do everything in Italian for Italian until you need English language stuffs.


Good luck.