Yesterday Microsoft released build 10565 of Windows 10 to Windows Insiders, introducing a bunch of new features and fixes, but also the usual "known issues" were present, as well as missing language packs for Cortana-supported regions.
Gabe Aul, the head of the Windows Insider group, tweeted that Insiders can look forward to grabbing the language packs later on today to re-enable Cortana in the supported regions.
It must be noted that, in the official blog post announcing build 10565, Aul stated that Russian and Turkish language packs wouldn't be made available until "later this week," so it's unclear if this has changed, or if those two language packs will be excluded from the rollout today.
Although the Insider builds support more regions than officially documented, with the recent addition of Australia, Canada, Japan and India (in English), there are still quite a few countries in which the personal assistant is unavailable. So far, it's unclear when other countries will eventually see support for Cortana, but Microsoft has stated that "Later this year Cortana will also be available first to Windows Insiders in Brazil and Mexico, and in French in Canada."
Right now, if you want to see Cortana in action in an unsupported country or region, you need to ensure that Region, Language and Speech (found in All Settings > Time & language) is set to "United States" in the Insider build, or wait for the supporting language pack to enable it.