The choice of language seems not entirely reliable. For example, it returns French results when told to search in English. And this from a feed set to French.
that’s because you likely misunderstand what this dropdown does: it isn’t a filter, language setting involves word stemming, stop words and such. if you take that into account with your keywords you should be able to search in any language and ignore the dropdown unless you actually need it.
i suggest you look up relevant documentation on postgresql.org (i.e. google to_tsvector and such) before any further questions here.
i.e. https://www.postgresql.org/docs/9.5/static/textsearch-controls.html and the page i posted recently about languages etc
iii. Is the language drop-down menu at all needed? Why not have a language: search operator?
most likely because to_tsquery() has this as a mandatory parameter and i figured why not expose it in the UI.