Song, poetry, and regional or foreign accents are only a few of the tough calls you have to make in the process of writing a novel. Do you include them or don’t you dare? Little effects and “writing within writing” can bring your imaginary world to life that much more, but it can also give it away as just another work of fiction by a writer who’s not actually very talented with words. Objectively there’s no way to know your talent. Is it a good idea to take the gambles?
It depends. If you’re drafting, then absolutely–take risks, break from what’s expected from you, go crazy! Write your whole book in verse, if you want. Editing is where it gets tough. You need to take your readers seriously if you want them to respect you as an author. Rhyming couplets does not guarantee a good fictional bar song any more than a cow guarantees the presence of the Wild West. People will see through your writing if it drops dramatically in quality for the sake of adding a poem or two or a strong Scottish brogue.
The best way to handle these sections in your novel is to show them to others, to people you can trust to give you an honest critique. Scribophile is a great place to check out if you don’t know anyone who could do the job. Otherwise, a rule of thumb that I usually stick to in the revising stages of my work is “When in doubt, cut it out.” (Catchy, ain’t it?) If you’re not sure you can pull something off, then you should probably work around the scene in a different manner. You could say the people at the bar were singing a rowdy song about whatever, or add that a character was speaking in a Scottish brogue. Some writers and literary critics even look down on others who include any irregularities of the English language at all in their work–so don’t worry about people thinking you’re not good enough if you back out gracefully.
Also, it’s better to write simply but well than to be overly elaborate. People like simple writing. They connect with it. They swallow it whole. If you show signs of straining, you can bet your readers will pick up on it. If you can write a conversation simply but convincingly, you’ve already done a better job than if you tried to write about a dramatic confession with all frills attached in your style and dialogue. That’s an area where far too many writers (including myself) fail. There’s a reason why people consider my children’s book to be my best work–it was simple and easy writing, writing where I wasn’t straining myself but sticking with whatever sounded like the easiest and most fun path. That’s the way it should be.
You don’t need to be a poet or a linguist or a world traveler to write an excellent piece of fiction. You just need to know your limits and work around them so gracefully that no one will even know they’re there.