Locus is not able to do offline navigation. This is because Locus is based on maps which consists of images. Let's say, it's just a photocopy of a real paper map. Other real navigation apps like TomTom and Navigon and Garmin and Falk and so on are using vector maps. In these maps are real "roads" and real "streets". So they know where they can calculate a route. Do you understand the difference?
The story with Google Navigator works as described... If you long-click in Locus on a point on a map, you can choose "navigation" and start Google Navigator with that specific point as target. If available, you can also start Navigon with that specific point. This feature is good for users which often use Locus and directly want to start their real navigation app without adding their target within the navigation app.
It is not possible until now, that you can use Google Navigator with offline maps. It has to do with the difference between vector and image maps (see above).
About the feature called "calculate route"... Open Street Map offers an online route calculation tool. Locus uses this tool. So if you want to calculate a route, you have to be online. But you can save a calculated route for later offline use. Please be aware, that only roads and streets from OSM maps are used. Some areas don't have good map coverage with OSM.
To put this feature into the right light... If a users uses "calculate route", Locus sends the actual location and the disered target to OSM server. The OSM server calculates a route and sends this route back to Locus, so that the user is able to see this route. It's just a track as you can draw by yourself. That's all so far.
But this means, that you only get directions for OSM-known roads, streets and places. This means further, that there are no voice commands available. And this means, if you took a wrong way, your route won't get automatically re-calculated.
Hope you get this right? This has also to do with image and vector maps.