The original Dynamics C5 product, was not related to Navision. In fact it was its hardest competitor on the Danish market in the early 1990'ies.
In 1986, a year before Navision, but after their first PC-Plus release, two Danish brothers Erik and Preben Damgaard lauch an accounting system called CONCORDE Finans.
The later release CONCORDE 4, becomes C4. After CONCORDE XAL has been released in 1991, as a direct Navision competitor, with a similar AL - eXtended AL, Damgaard keeps the old version alive for their SMB customers. In 1995 they release C5 for Windows. XAL again becomes Axapta or Dynamics AX. So C5 is really more in family with AX.
The original C5 platform from 1995 continues and is updated until Dynamics C5 2012.
The first "true" NAV version of C5 was Microsoft Dynamics C5 2014. It was released integrated into Dynamics NAV 2013 R2 in October 2013, as the "mini-app" or the small-business role-centers.