Can't update custom image field from API

[joe_g]

I've got an image field with an added custom field "caption", this caption is multilingual. I'm trying to update this field from the API, but I'm not managing.

Even though I set a language to non-default the data is saved in the default field.

For example if set the language to Spanish

$user->language = $languages->get('es');

Then I set the caption ($block is a repetear matrix)

$block->of(false);
$block->images->first->caption = 'Spanish Caption';
$block->save();

the default caption contains 'Spanish Caption' and the spanish version is empty.

Any ideas are welcome ?