If you have errors similar to INSERT INTO sylius_shipping_method_translation (name, description, locale, translatable_id) VALUES (?, ?, ?, ?)' with params [null, null, "lv", 1] error after services.yaml changing locale , also with another Sylius tables, try to manually add these records in database.
For example, if you have problems with product db table, go to admin/products/{id}/edit and add translation to your main locale, do it for every product.
It occurs when you switch the main locale, then adding something to cart (in another words create an order). Order entity has relation to Shipping method, which has relation to Shipping method translation entity, which doesn't exists with needed locale. So Doctrine thinks it should persist before creating an Order, so there occurs an error.