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
Merge pull request #1878 from piquadrat/feature/render-placeholder-language

Added option to render_placeholder tag to set language
Merge remote-tracking branch 'upstream/develop' into develop