Hey there, welcome to comics.
Heads up, this probably won't work. If you have a good comic you might have people who are interested in the comic itself, and maybe in the world and the characters, but there probably won't be that many people who are interested in reading the novel. What's more likely to happen is that you'll just have a comic that you also have to try and get people to look at.
Not to say that you shouldn't make a comic, comics are great! But they're not a good tool for making money.
If you're still interested in making comics, or adapting your novel into a comic, for artistic reasons, then feel free to read on.
I've never adapted a novel, so I'm mostly coming from a place of just having experience with making comics in general (though I have adapted poems into comics before). Ill tag @punkarsenic. Their comic is based on a prose version of the story that they wrote, and they might have some specific insights into that process. I've also heard of other people who use novels/prose as the first draft before making their comics, so if you ask around more you might find other people who can give some advice.
Colored speech bubbles aren't uncommon, however they're usually used as a way of influencing how the "voice" of a character is perceived, or just as a stylistic decision. As long as you keep in mind basic color theory and apply your sense of design (as you should do with every element of your comic), I don't think it would look bad. That said... I think there are probably better ways to show that a character is speaking a different language. One method that I'm fond of is to just put the other language in the speech bubble. In Maus and Persepolis, when characters are speaking different languages, but the audience still needs to understand whats being said, (their dialogue is written in parenthesis like this, and then the narration explains that they were talking in french or whatever.) There are probably other ways you could approach it as well.
This is one of the main creative challenges of adapting any work. I can't really tell you the answer. Go with your gut, and do your best to make a cohesive story.
That said, there are definitely things that comics are better at adapting than other things. The obvious example is anything with cool visuals. But also, comics are really great at showing conversations, and how the people having those conversations relate to their environment. Things that comics aren't good at (in my opinion) would be stuff like internal monologues and expositional narration. That said, Persepolis (which I mentioned earlier) is basically one long internal monologue, and persepolis is great. Comics are a really weird medium.
The best advice I can give here is to just try your best? A method that concept artists like to use is they'll make giant reference boards of relevant images. For example, if I was going to try and collect reference for a giant spider robot or something, I would collect a bunch of references of A: spiders and B: Mechanical Engineering. and then I would probably die because spiders are creepy and mechanical things are a pain in the ass to draw.
Some general advice for making comics that might help you out:
1) Read Understanding Comics by Scott McCloud if you haven't. Just do it. Its the best book for learning about all of the theory behind how comics work, and does a great job of breaking down and giving examples of a lot of the unique capabilities of the medium. As far as I'm concerned it should be required reading for anyone interested in the medium. It's not expensive, but if you don't want to spend money you can find PDFs online pretty easily.
2) Make a different comic first. This sounds like a huge project, its probably more than you can chew, it's more than a lot of people who already make comics can chew. I would start off with something smaller to test the waters first. Maybe instead of setting out to adapt the whole novel just adapt a scene, or write a small side-story, or just do something entirely different. You'll be glad you did. A small project in this case means something that you can get done in a month, chances are it will be less than 20 pages, probably less than 12.
3) Start studying how to draw, and maybe even a bit of graphic design, if you haven't already. A lot of people make the mistake that cartoons/comics are easy for some reason? They're not. You need a pretty specific skill set, and in order to get any sort of attention you need to be pretty competent. Don't take this to mean that you have to be good at drawing before you start making comics, but don't underestimate how difficult what you're planning on doing actually is.
