You may need to do a FILE COMPARE between the Language Module's files, and their counterparts on the 1.3.9 version.
As there will only be English files in the 1.3.9 version, your aim is to see if additional DEFINES have been added, or any code changes have been applied.
The CHANGELOG for 1.3.9 shows that the following files have indeed been changed:
/admin/includes/languages/english/attributes_controller.php
/admin/includes/languages/english/categories.php
/admin/includes/languages/english/countries.php
/admin/includes/languages/english/coupon_restrict.php
/admin/includes/languages/english/developers_tool_kit.php
/admin/includes/languages/english/document_general.php
/admin/includes/languages/english/document_product.php
/admin/includes/languages/english/extra_definitions/linkpoint_review.php
/admin/includes/languages/english/modules.php
/admin/includes/languages/english/options_values_manager.php
/admin/includes/languages/english/product_free_shipping.php
/admin/includes/languages/english/product_music.php
/admin/includes/languages/english/product.php
/admin/includes/languages/english/reviews.php
/admin/includes/languages/english/store_manager.php
/admin/includes/languages/english.php
... so clearly these will need to be looked at.