Added option to render_placeholder tag to set language

This is helpful in cases where the content of a placeholder in a specific
language should be displayed, independent of the user's language.
some documentation about the language argument for the render_placeholder tag
Fixes 404 when moving page.
Merge pull request #1878 from piquadrat/feature/render-placeholder-language

Added option to render_placeholder tag to set language
Merge pull request #1880 from czpython/patch-8

Fixes 404 when moving page #1875