As stated and shown in this post by @chongoblog, Super Monkey Ball Banana Rumble features it own version of the characters of the Latin alphabet and Arabic numerals. These characters are used to decorate the world and convey messages and info that can be converted one-to-one (mostly) to English. It should be noted that their written word largely lacks punctuation.

Perhaps the loudest hint that this diegetic writing has direct meaning comes from the cosmetics/clothing you can dress playable characters with. The "Loungewear" item differs slightly for each character, and features their name on the front. Great reference for vowels!

Here are some more examples from other cosmetics:

The numerals are bit harder to pin down, as examples of them are fewer and further between. Aside from the "24 HOURS" sign in the background of Neon Arena and the year "2023" seen in some Rose Garden levels, here are a few more sightings:

The full-page text-only entries in Palette's diary (diaries?) repeatedly appear throughout the course of the credits sequence. The pages that feature a photo and/or drawing appear only once, but are unfortunately all partially obscured. The full-page entries can initially be seen at the start of the credits, when Palette's diary opens with a flurry of pages. However, among that flurry is an entry that does not seem to reappear later in the credits sequence:

Palette's name (and noggin) are also on the back of the diary at the end.

If I may digress for a bit, who's this yahoo???

This character's mug is also featured on the koban that lay in the background of Golden Temple and on the corresponding travel brochure. Doesn't seem to resemble any of the Super Monkey Ball Adventure characters or anything. I wish they'd show us YanYan's dad…

Additional examples of the the written language:

Here are some miscellaneous text samples I whipped up: