Update: Looks like German and French have been live for a while now, though most of the other languages are pretty new. Again, if you're not seeing this feature yet, it's rolling out, so it may be days or even weeks before it's fully available.
If you're suddenly noticing that Google Now is telling you that you can activate it using the "OK Google" command, good news: you [probably] aren't going insane. We've received multiple tips that the previously US-only, then US, Canada, and UK only, feature is rolling out to a wider number of linguistic locales. Some of these have been in testing for weeks or months, so if this isn't news to you, you were probably one of the lucky ones. You can see the complete list of supported languages below.
- English (now supports all accents / dialects)
- Spanish (Spain and Mexico only)
- Portuguese (Brazilian only)
You may need to be running the most recent update to the Android Google Search app (3.5) in order for this feature to activate, so if it hasn't rolled out to you, you can download the APK right here. You'll also need to make sure it's enabled in Google Now's settings, which you can find under the voice area of the primary app settings. The feature appears to be rolling out gradually, like all things Google, so be patient if you're not seeing it just yet.