While I cannot reproduce the exact issue with my current locale/language settings, I can reproduce something that is likely related.

1. Name an object ж
2. Add > Curve > Sapling Tree Gen
3. Switch settings to Leaves
4. Query the Leaf Objects and select the one named ж

The name is not properly displayed for the enum and every time it is selected there character changes (it's either blank, p_, ðZ, Đ]).

Looking further into this, it seems to be caused by the dynamic construction of the enum. The leafDupliObj EnumProperty is defined as:

leafDupliObj: EnumProperty(
    name='Leaf Object',
    description='Object to use for leaf instancing if Leaf Shape is DupliFaces or DupliVerts',
    items=objectList,
    update=update_leaves
    )

The enum items are dynamically populated based on the return value of objectList implemented in AddTree (__init__.py).

def objectList(self, context):
    objects = []
    bObjects = bpy.data.objects

    for obj in bObjects:
        if (obj.type in ['MESH', 'CURVE', 'SURFACE']) and (obj.name not in ['tree', 'leaves']):
            objects.append((obj.name, obj.name, ""))

    return (objects if objects else
            [('NONE', "No objects", "No appropriate objects in the Scene")])

According to the Python API docs using a callback for items is known to cause issues when not keeping a reference to the returned strings:

There is a known bug with using a callback, Python must keep a reference to the strings returned by the callback or Blender will misbehave or even crash.

Dear Robert

I found issue of my problem

If I use cyrillic letter like "Ж" on my ICOsphere, this problem appear
BUT if I use cyrillic letters on any plane - all correct

Please, check my file.
https://drive.google.com/file/d/1yCjH3MmheHVn8IXVZt1LBEUQguZmwV9p/view?usp=sharing