First one (tested in N++):
Search: \$lang\['(abc)'\]
Replace: lang\('$1'\)
Second one (also tested in N++):
Search: \{\$lang\['(abc)'\]\}
Replace: ".lang\('$1'\)."
The back reference mentioned by devnull is the content of the (abc) parentheses. The parentheses capture abc into Group 1. That content is referred to in the replacement as "$1". You may like to read all about regex capture.