fix altnames functionality in ftml_builder.py

[bobh0303]

According to the code, FTML_Builder's FChar and FSpecial classes include:

        self.altnames = {}  # alternate glyph names.
            # the above is a dict keyed by either:
            #   lang tag e.g., 'ur', or
            #   feat tag and value, e.g., 'cv24=3'
            # and returns a the glyphname for that alternate.

However in readGlyphData() it looks like only the latter key (feat tag and value) is actually added to the dictionary -- lang tags are not added.

Either change the code or change the comment.