Every Garmin Nuvi has a TEXT folder containing the non US languages .glx files.
In these files are the translations of the "firmware US English" strings' text that will be shown on the screen, when the strings are called up.
Inside your pk_files66.zip is a Garmin/Langs/ folder containing just two language files, es_US.glr (American Spanish, pn 006-D2244-03 ver3.90) and fr_CA.glr (Français Canadien, pn 006-D2244-03 ver3.90) but they are in binary format, not in xml format as in Nuvis.
Maybe your device doen't have a Text folder and .glx files.
Your strings in the fw_all.bin start with STR_ because it's an OEM device.
There are 3 strings on the start screen, "WARNING" in red, the "warning text" and the "AGREE" on the bottom.