Your error message seems to indicate that you have product(s ) not belonging to any category. I can fix the code so it won't stumble over this.After some editing I tried to Import but the following error halted the process:
Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
Error No: 1064
INSERT INTO `product_to_category` (`product_id`,`category_id`) VALUES ;
I have only tested the Export/Import feature for the default language. Assuming you have exactly the same products and categories for each language, you'd have to run the Export/Import separately for each language if you change language-specific descriptions, and switch the default language between each Export/Import run via OpenCart's /admin/index.php?controller=language function.
During the import, the language-specific descriptions (e.g. product descriptions, etc) are only replaced for the current default language which is checked against the language_id column from the spreadsheet.
Most product and category fields are language-agnostic. E.g. if you change prices only then just one Export/Import cycle is needed, regardless of the default language set.