Hi all, thanks for all replies.

> indeed. `` and '' are (in tex) pseudo ligatures and not really a
> portable standard, certainly not now that we have utf8 as input (in a
> similar fashion users who need accented characters will not use \"u and
> \'a any more)

In fact, I don't know how to input left, right double quotation
marks and accented characters directly with keyboard.
It is easier for me to input {``}, {''} and \"u etc than
to input utf8 characters with keyboard.
I don't say to make tex-style ligature available by default
but only to make it available under an appropriate setup.
Even this is not suitable for ConTeXt?

Best regards,
Atsuhito Kohda