Deprecate Language::setCode as public method

Deprecate Language::setCode as public method

setCode changes the language code for the Language object but it also
replaces the whole language codes for all Language objects.

> $lang = Language::factory( 'fr' )

> $lang2 = Language::factory( 'fr' )

> $lang->setCode( 'it' )

> print $lang2->getCode()
it
> $lang3 = Language::factory( 'fr' )

> print $lang3->getCode()
it

Better assign a new Language object.

Also add more tests for Language::equals.

Depends-On: I61439bac82021344c3f9a6056cccd937b3450af2
Depends-On: I2d9e551d6eb33f28f42aeaf48160eba21b83881f
Change-Id: I201b479f58e63c9c40fb8a3ec9575a551fb35235