Modify Craft locale inside Element API plugin?
Craft::t() has a whole bunch of parameters you can pass into it.

public static function t($message, $variables = array(), $source = null, $language = null, $category = 'craft')


It sounds like what you're looking for is the 4th one.

Craft::t('my string', null, null, 'es')


Where 'es' would be a target locale other than your main one.