a better solution for my if-condition

You're missing the case where Minuten_dezimal is exactly 22.5. I'll assume you want to treat that the same as > 22.5.

Something like this would work:

var clockTime = System.getClockTime();

var hoursToText = [
    "zwölf", // 0
    "eins",
    "zwei",
    "drei",
    "vier",
    "fünf",
    "sechs",
    "sieben",
    "acht",
    "neun",
    "zehn",
    "elf" // 11
];

var hours = clockTime.hours;

// Increment hours by 1 if mm:ss >= 22:30
if (clockTime.minutes > 22 ||
    (clockTime.minutes == 22 && clockTime.seconds >= 30)) {
    hours = hours + 1;
}

// Ensure hours is between 0 and 11
// 12 -> 0, 13 -> 1, etc
hours = hours % 12; // x % y gives you the remainder of dividing x by y

var stundentext = hoursToText[hours];