Why don’t we say Kaddish in Hebrew so that it is more easily understood?
The saying of Kaddish in general and by a mourner, in particular, has great significance in Halacha and more so in Kabbala, which is beyond the scope of this answer. Nevertheless, I will give a brief explanation to answer your question:
Kaddish was originally written in Aramaic for a variety of reasons: so that the angels should not understand and become jealous of this special praise we give to Hashem; because it causes Hashem to remember the destruction of the Beis Hamikdash and we don’t want the angels to be aware of this and use the opportunity to speak negatively; because that used to be the common language spoken; according to the Zohar we are breaking the force of evil found in the secular language and utilizing it to praise Hashem. In any event, that’s the way it remained in our davening.
אמירת קדיש נת’ בטור או”ח סימן נו ובזוהר פ’ תרומה דף קסט, ע”ב מובא בבית יוסף שם.