"Either the OP is being ridiculous, or her mental health issues mean she’s over reacting"
Reactions due to a MH problem are not 'overreacting', they are the symptoms of a person suffering a MH problem.
This visit sounds poorly planned (by the DH) - surely he could have made sure to have been there for the whole of the first day at least - think of all the 'house rules', information, baby's schedule, how to turn the telly on all of that, that he could quickly and easily have availed his mother about.
Instead he's leaving it all to OP when he knows she is stressed out by it - how bloody selfish is that?!
Having said that, I'd have go with what PPs have said and use Google translate and think of what things you think you'll want to say to her "Can you change baby's nappy", "Can you make me a sandwich whilst I feed baby", "Do you want a cup of tea?", "I'm going to take baby to the park, would you like to come".
As well as "So lovely to meet you and welcome", you know, that sort of thing.
Could you get your own mum over on the first day to help reduce your tension a bit?? Are there friends you can have round or a coffee morning or something that you could take her to - again, possibly reducing the anxiety and isolation of it just being the two of you??
I find people are generally quite dismissive of anxiety offering up 'why don't you just' and other such useless sentences. No one with anxiety wants to feel the way they do, or carry out behaviours that the anxiety leads them to do!