You don't say what the program versions is, latest program version on regular release is 2015 10.3.2225.
The only program update released recently, today is a beta version 2015 10.4.2227.
What version do you have ?
If it is the beta version mentioned, then not all languages may be supported.
You should be able to download additional languages (assuming you aren't using the beta build) from the avastUI > Settings > General - Language - Install Additional Languages.