Based on this report
https://github.com/koreader/koreader...ssue-585591017 neoreader is using stardict dictionaries, stored in /sdcard/dicts.
It seems that neoreader is not using common android intents to do dict lookups. You can verify this following steps 1 and 2 from the following answer:
https://github.com/koreader/koreader...ment-602112761
The way android apps do intercommunication is via a set of intents, like SEARCH and SEND. Each dict app register itself as able to resolve one or more of these intents, so it is available on the app picker.