In Preferences -> Segmentation Setup
Choosing a Language Name and then adding a new rule, then clicking OK,
The new rule is often not saved - or rather, a new rule is saved, but with the default "." "\s".
A workaround is to add the rule, click on another previously saved rule, and then click OK. That will save the new rule, but it is clumsy.
This is fixed in trunk, r10138.
Fixed in the released version 4.1.3 of OmegaT.
Didier