Hi,
You try to clear your cache, or search if something is wrong with the configuration of your namespace.
To find out if MODx is reading his file "lexicon" place this code at the beginning of your file
exit ("I am here brother!");
if this goes on your screen right block, then place the "exit code" at the end of the file.
If you see the message again then the error is
this in the name of the array indexing $ _lang.
But if you do not see the message, surely
There is an error of concatenation of text in your code.
Regards.