It is not hard to work out short phrase in the native language to get started including particularly the courtesies that we all use to start a converstion. They my be symbolic but with out them you can appear rude in any language.
People soon switch to English if you have difficulty. If they don't speak English you can both have fun and lots giggles with gestures, pictures, pointing, etc.
Much as I like France there are some French people (only some) who have chip on their shoulder about language and can be rude.
In the rest of Europe the attitude is more "the guest can't speak my language and I can speak thiers so we will use theirs".
To be fair to the poster it was not a shop or ticket counter but the information counter at a main railway station in the worlds most popular tourist destnation and a better response could have been expected.
But back to the original topic. I'd go BA and change at LHR you will transit in the same terminal (5). The whole day will be travel anyway from checkng out, getting to an airport etc.
I am doing somthing similar March going Berlin to Stavanger via LHR. Making it part of my OW Aus to Europe return ticket was cheaper than buying a direct flight seperatly.
