py.mk: allow translation to be overriden in GNUmakefile
py.mk: allow translation to be overriden in GNUmakefile

I like to use local makefile overrides, in the file GNUmakefile
(or, on case-sensitive systems, makefile) to set compilation choices.
However, writing
    TRANSLATION := de_DE
    include Makefile
did not work, because py.mk would override the TRANSLATION := specified
in an earlier part of the makefiles (but not from the commandline).

By using ?= instead of := the local makefile override works, but when
TRANSLATION is not specified it continues to work as before.

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython