While German isn't known for its slang compared to English, younger people and social media users have started to adopt slang or humorous ways to say happy birthday in German. For instance, you might see "Alles Jute!" instead of "Alles Gute!"—a playful way of misspelling that mimics a Berlin dialect.
Some even say "Häppi Börsday!", mimicking the English phrase but spelled phonetically in German. These are usually used among friends who are joking around and shouldn't be used in professional or unfamiliar settings. Still, it's fun to see how happy birthday in German (https://www.gradding.com/blog/german/happy-birthday-in-german) has evolved with the influence of internet culture and youthful expression.