Locale: changes display of dates, times, numbers, names, and addresses.  Notice the difference in date formats between the US (MM/DD/YYY) and UK (DD/MM/YYYY).

Language: determines which language components (fields, setup menu, etc.) are displayed to the user.  Notice the difference between English and French.

Time zone: determines the offset used to display time references in Salesforce (similar to changing the time zone on your computer).

Currency: can only be selected when multiple currencies is enabled, and will determine the default currency for records created by this user.  If currency cannot be selected on the user record, then multiple currencies is not enabled and the organziation’s currency locale will be used to determine how currencies are displayed for all users.

There are multiple tiers of language support in Salesforce.  The tier of language support will determine which standard components are translated by Salesforce.  Components that are not translated will use the organization’s default language.

• Fully Supported: translates all standard fields, objects, and setup menu
  • Example: French | Tabs & Fields Translated | Setup Menu Translated
• End User: translates all standard fields and objects, but not setup pages
  • Example: Hungarian | Tabs & Fields Translated | Setup Menu NOT Completely Translated
• Platform Only: no translation of standard objects, field, or setup menu (custom translation only)
  • Example: Icelandic | Tabs & Fields NOT Translated | Setup Menu NOT Translated

Custom fields and objects must be translated using the translation workbench (otherwise the untranslated value will be displayed).

Data (e.g. the name “John Smith” in a lead record) is largely not translated (one exception is picklist values, which can be translated).