issue #990: provide a more flexible way of finding translated content

This commit intorduces the TRANSLATIONS_PATTERN conf var
should be changed to a new default in v7
changed logic of locating original files and their translations

as it is hard to reverse-determine the original_name of a translated
post, rather remove translations from list of all files
rooted out other assumption about pattern file.ext.lang
flake: removed extra space after operator
docs: translated pages -> translated input pages

to reflect what we are talking about
update docs/manual.txt for TRANSLATIONS_PATTERN
the new recommended TRANSLATIONS_PATTERN will be path.lang.ext

+ a few modifications by Kwpolska