Seems like the verb 'to try' is similar in many languages
This one will come in handy in conversation with my future Polish wife.
You can try, but you won't succeed. Trust me :D
The boss would like to speak to you, speak slowly, I can't understand you when you speak Polish etc.
I believe it's more formal than talk here.
Yes it can be used formally but also very informally:
I am trying to speak would also fit the bill as it is more often used talk.
Is 'speak' used often for having a conversation?
Yes. I would in fact label it just as common as "talk" in this context. For example, if you're really irritated with somebody:
"AHEM, who told you you could speak?? I'm trying to talk here and no one is letting me!"
I hear you speaking and I concur. Talk about getting confused if you don't speak English, when you wish to talk to someone who does/
That's any married man when his sister in law comes to visit his wife. An exercise in futility.