Hi, I would like to ask which of the titles sounds better?
Let's talk in Lithuanian
Let's speak Lithuanian
I think that the first sounds more appealing.
'Let's talk in Lithuanian' puts the emphasis on the idea of 'Let's talk'. It reads like a friendly invitation to have a conversation.
'Let's speak in Lithuanian' implies a decision to use Lithuanian as opposed to another language. For example, 'Let's speak in Lithuanian ( not Russian or English or any other language) (so that other people won't understand us)'. It doesn't sound quite so welcoming.
Just my impression!