This is important for me too. The store was released in one language. Now I have added a translation. Existing customers want to receive emails and notifications in the new language, but since they made a purchase in the original language, their customer_locale is stuck in the original language.
Please let me know how the customer_locale property can be corrected/updated.
I reached out to Shopify's customer support team, and after a long discussion with their engineering team, they told me the customer_locale value is set based on the site language of their last registration/checkout. I was able to confirm this by creating two new accounts, registering with the site set to Japanese and English. I then went to the Customer Management screen, and confirmed their language setting by filtering the list by specifying "More filters->Customer Language".
What sucks about this is that once a customer_locale is set, the only way they can change that value is by making another purchase. To get around this problem, I have registered a free digital download product called "Set notification language to [desired language]". By instructing customers to download this file by setting the site language to the language of their notification choice, customers can update their customer_locale value.
This is a hacky hack, but at least it is the first workaround I have found on this site. I'd love to hear more elegant solutions if people have one.