First thing - the box around inset has not length of the whole line. Second - try to write some text and delete it - the box is not correctly repainted and the deleted letters remain displayed.
Known problems... nobody has paid enough attention on them though.
2. I would like to differ translation for the same string 'Listing' according to context where it appears. I know there is [[context]] mechanism for it in .po files but dont know where I found which "context" string should I use.
No idea.
2. Seting advanced options was unintuitive to me - e.g. type "fillcolor", now help wants me to add parameter, however I was not able to find howto put the parameter there to satisfy validation.
Something like fillcolor=\color{red} You will have to consult listings manual for the exact usages of the parameters. You are also welcome to submit patches that add hints to specific parameters (read src/insets/InsetListingsParams.cpp) so that others do not have to. :-)
3. Rather TeX question - when typewriter font family is set, keywords are no more bold (maybe it is not possible to typeset this font in bold, dunno)
This is quite likely.
4. I was not able to produce tabelators while editing. I dont know whether this could be easily implemented, but to work with source code without tabelator is very annoying.
This is also a known bug (have to check bugzilla for bug number) that is, however, nontrivial to fix. My suggestion would be writing code in an external file and include it in insert -> file -> child document -> listings. With options firstline and lastline, this is quite convenient. Cheers, Bo