Amend commit 75153f7b76 " Fix clash with locale variable" by Andreas Schwab.
Use DOCLANG instead of INFO_LANG for renaming, as documentation does not
produce only info output, and DOCLANG is iterating over DOCLANGS.
* Makefile.in: Rename 'LANG' to 'DOCLANG' to avoid clash with well-known
environment variable.
* doc/misc/Makefile.in: Adjusted accordingly, rename 'INFO_LANG' to 'DOCLANG'
where Andreas had already renamed 'LANG'.
* doc/translations/fr/misc/ses-fr.texi: rename 'INFO_LANG' to 'DOCLANG' in
explanatory comment.