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!