1) while drafting this plug-in, @red51 asked me not to rely on any external or third party library or plug-in; as the pug-in is going to be kind of "official".
In fact, after some discussion, a decision was made to even duplicate a part of RW-Gui into this plug-in, rather than require RW-Gui itself.
So, I assume it is not the case to rely on SprachAPI. However, this does not exclude support for any language; the plug-in is shipped with a few languages, but anyone can rather easily add any other language: the locale/readme.txt explains how.
2) Bug: Am I correct in assuming that that bug shows up in Single Player? Debugging SP is difficult, but I suspect there is a bug in RW Single Player code which does not call some of the initialisation steps of plug-ins.
The workaround I found is to issue the reloadplugins console command right after starting an SP session. With this, plug-ins work correctly and the bug you describe does not happen. Without this, for instance, the GPS does not even shows its small info window.
I'll check with @red51 if my diagnose is correct. For the time being, the workaround should work.