On commerce_product_product_form the language code will be empty if the product is a new entity. The form used to take this into consideration with
$language = !empty($product->language) ? $product->language : LANGUAGE_NONE;
However, with the entity translation integration, it now expects to get the language from
entity_language, which may return
$langcode = entity_language('commerce_product', $product);
A simple check for null with a fallback to
LANGUAGE_NONE should be added to ensure that a language code is always set on the form.