Update: Since writing this article, it would seem as though English UK is now supported. I’ll leave the article here as it may be of relevance to other languages.
I read about an issue with the constant-listening feature of the Nexus 5, where saying “OK Google” would not work if you had any language other than English US as the system default. In having this language set, I have come across a number of issues, such as Amazon’s Android app loading the US store as well as having the default input language set to English US.
The default language issue was an easy-enough issue to circumvent, as it is possible to pick other keyboard languages, but there really is no way around the Amazon app issue. In searching through the options though, I have managed to change the “voice search” language to English US, whilst leaving the rest of the system in English UK. This means that speaking “OK Google” does work and loads the search window, but that other apps and settings which rely on the system language also work.