Yesterday I released a new version of EXT:ajaxselectlist. The extension now supports TYPO3 v8 and TYPO3 v9.
Due to several changes in the TYPO3 core I first had to restore the multilingual support. When updating, you'll need to add a new input field to the List.html template:
<input type="hidden" name="id" value="{currentPageID}">
This solution is to be understood as a temporary workaround.
My goal is to change the Ajax call for the next major version (5.0.0) to the newly introduced PSR-15 middlewares. I have to find the time to deal with this topic, though. This planned version will then run under TYPO3 v9 and the upcoming version 10.