Right here:
The slide show is actually shared across Ubuntu and its flavours, it would be worth reporting the slide transition as a bug on Launchpad.
Translations for the login screen are here:
Later this week, we should be launching our new website which has a section listing all the programs for translating. I actually missed off Slick Greeter, so thanks for the nudge
It's definitely not the RTL code since the version in the daily ISO is still the "stable" version of Welcome. The new code is still in "beta" channel of the snap which can be activated with this Terminal command:
snap refresh ubuntu-mate-welcome --channel beta
It is likely that Welcome did crash the system. I haven't been following the bug closely, but it might be due to 3D acceleration in VirtualBox.