One of the known issues with the latest Windows 10 Mobile 10080 build that was released yesterday is the issue of Cortana crashing constantly, making her virtually unusable.
As Gabe Aul mentioned in the release notes for this 10080 build, this issue happens when you are not using the default region/language settings that came with your Lumia device.
As Cortana will only under 2 requirements
1. The default region/language is one where Cortana is currently supported (UK, USA and others)
2. The default region must be set on your Lumia for Cortana to work.
I encountered this problem, I have a Lumia 930 that's from Germany (with German product code), it's default region is therefore German. With Windows Phone 8.1, you can change the region to USA to allow you to use Cortana. However, in this technical build, if your region is set to USA but the default is German, Cortana will constantly crash (She will not work) and if you set the region back to default German (in this case), Cortana will not crash but give you a message that she isn't supported in your region.
So how do you fix this? Simple, flash a UK firmware on your Lumia, currently the UK firmware is the only english speaking firmware that supports Cortana. Since the default region is now UK, I was able to use Cortana after installing the 10080 build.
You can learn how to flash a ROM on your Lumia by following these easy step-by-step tutorial from the guys at XDA developers.