fix #1544 -- make INDEXES_* strings translatable

cc @felixfontein
I'm wondering a bit why we don't already assign ```kw['indexes_pages']``` to what currently is ```kw['indexes_pages'](lang)```. Then the (for example) english version will only be recompiled if the english text changes, and not if any other translation changes. Since the string is here inserted into ```kw``` only after ```lang``` is fixed (because a copy of ```kw``` is created in that function), that should work.