strings.xml is not in the navigator folder. It is in the internal apk structure. An apk is nothing more then a zip file (just like a java jar file by the way). If you unzip the apk you will find in res/values-fr the strings.xml.
Most likely you can not update it and zip it back as the MNF apk will be signed. Rezipping it will change the CRC and make the signing invalid. You might try the apktool.
- decompile: "apktool d navigator.apk"
- modifiy the res/values-fr/strings.xml
- recompile "apktool -c b navigator"
- copy the recompiled navigator with original signing files (-c) out of the navigator/dist folder.
Or better: simply wait till the translation site is activated again. :)