Thanks for all the feedback sulu, it's much appreciated!

Now for the individual issues:

bad route/route with bad segments
As rinigius already mentioned, waypoints are a reasonably good solution. Valhalla and many online services support them and modRana actually does too! So what's remaining is support in the (Qt5) GUI, which is definitely planned (and quite a priority actually).

There are also other (possibly complementary) options. One of them is returning multiple routes (from different backends or with different settings) and letting the user select one. Also it could be possible to provide the router with more data and increasing the chance of a good route being found. This could be combined with the first option.

And if you are on a bad route or conditions change - modRana does have automatic rerouting and it should be enabled by default.

turn-by-turn voice issues
There are multiple issues and ongoing efforts to fix them. As for the voice quality overall, rinigius did a significant amount of work in this area (thanks! ) by porting multiple new & likely better than espeak TTS engines and then helped them to be used by Poor Maps.

I should be able to reuse much of this in modRana, but this requires changes in the turn-by-turn navigation logic. At the moment voice messages are generated and "spoken" immediately when modRana detect they are needed.

To make use of the new TTS engines and the need for people to install obscure packages manually, the logic needs to be changed to pre-generate the messages to temporary files first and then just play those files (via QML) once needed. This could also make recorded voice sample based navigation possible (IIRC Marble has a good collection of multilingual navigation voice samples).

As for local name vs instruction language missmatch - this could be a hard nut to crack as rinigius mentioned. I think I should definitely expose any language selection settings for the routing engine and/or the TTS engine - that should help in many cases.

Also - it it possible to detect the language for place names in Valhalla output ? If they could be tagged by language (or even just country) it might be possible to set a different language in the TTS engine just fort he place name (at least espeak can do that).

Actually, modRana can do this in a limited capability already - there is an option (only in GTK2 GUI, not yet exposed in the Qt 5 GUI) to consider words with cyrilic to be Russian words, so that espeak will speak them in Russian. If we can detect the language by more robust means, this technique could easily be used universally.