I have the following XAML (simplified and emphasied for readability):

XAML:
<UserControl x:Class="FantasiaPhone.BrushAndColorControl"
...
xmlns:t="clr-namespace:Translation">

...
<TextBlock Text="all colors" FontSize="14" VerticalAlignment="Bottom" HorizontalAlignment="Center" TextWrapping="Wrap" TextAlignment="Center"
t:Translate.Text="allColors" />
...
</UserControl>

That I load like so:
Load code:
NoesisXaml xaml = (NoesisXaml)UnityEngine.Resources.Load(Path.GetFileNameWithoutExtension(xamlPath));
xaml.LoadComponent(o);
Debug.Log("Loaded");

I also have the Translate class:
C# t:Translate code:
public class Translate : DependencyObject {
...
public static DependencyProperty TextProperty = DependencyProperty.RegisterAttached(
"Text", typeof(string), typeof(Translate), new PropertyMetadata(string.Empty, new PropertyChangedCallback(OnTextChanged)));
static void OnTextChanged(DependencyObject target, DependencyPropertyChangedEventArgs args)
{
Debug.Log("TEXT CHANGED");
}
...

I can load the XAML fine, display it fine, and OnTextChanged is called OK. However, I expect to see that the text changed event is called before "loaded":
Expected debug log:
TEXT CHANGED
Loaded

Actual debug log:
Loaded
TEXT CHANGED

After I call LoadComponent() I immediately go through the list of controls and update their text with translation. Since this is already in place I don't want to use bindings (they are also slower.)

Why is the above order of operations happening? How can I reliably determine that all controls have their t:Translate="..." handlers executed?

If I understood you correctly, the issue is that the attached property binding happens with some delay and you expect it to happen right after xaml.LoadComponent(o).
As a workaround you could try calling UpdateLayout() method for the loaded control right after xaml.LoadComponent(o).

Thanks! Figured it out. UpdateLayout() doesn't trigger as well.
The XAML component has to be assigned to the visual tree, otherwise it will not trigger the static property handlers. I changed my translation code to refresh after adding the control to the visual tree and all is well now

UI elements are not loaded until they are attached to the View as you discovered.
This is when you will start getting notification of property changes.