There are a number of paths you can take, some of which may be easier than others while others may not get you where you want to go.
Create Your Own Full Fledged Language!
Obviously, this is the way of worldbuilder cum glossopoet. Channel your inner Tolkien and make your own fully functional language complete with grammar, lexicon, syntax, texts and writing systems and all of that diachronically (starting from an older level and "evolving" the language through time). Die-hard fans of your work will love you forever if you go this route!
It sounds like this is not the way you want to go, but it is loads of fun!
Make a Language Sketch!
This is just a step below the first option. Here, you're creating enough grammar and lexicon and example sentences that fans of your work could write simple in-world messages and bits of lore. They can also use that knowledge to decode whatever hidden gems you place in your work, be it road signs or billboards or whatever.
A good example of this kind of language invention in comics is Herge's Syldavian language in the Tintin series.
Make a Naming Language!
A naming language is basically a list of (say, 100 or 200) highly important words & phrases that you plan on using in your work. If it's sword-n-sorcery fantasy, you probably won't have a word for starship, but you might have words for the various races of beings, curse words, expressions of surprise & anger, greetings, benedictions, and the like.
For the purposes of a visual work, even a naming language should be given its own writing system.
Do the Work Yourself!
This may involve an amount of study in how languages work (grammar) and sound (phonology) and are written. There are print resources (Language Construction Kit & Create a Language Clinic) as well as online resources (Conlang List & CBB for example) readily available to help you undertake the task. Some are geared towards writers (probably what you're interested in) while some are aimed at the hobbyist. Constructed Languages Stack Exchange exists to help you if you go this route.
Hire Someone to Do the Work for You!
Glossopoetry is an imaginative art. Making a good language is also a craft that takes practice and skill. If you think you can not or do not want to undertake that part of your project, you can consider extending a commission to a language inventor. This is how Dothraki got invented for Game of Thrones. The Language Creation Society exists, in part, to help connect people looking to have a bespoke language made with people who can make those languages. They can also help with questions of legalities, remuneration, properly crediting and so forth.