Navigation And Bottom Font Style :

Bottom :

THEMES :

<style name="Widget.BottomNavigationView" 
    parent="Widget.Design.BottomNavigationView">
    <item name="android:fontFamily">@font/your_font</item>
</style>

<android.support.design.widget.BottomNavigationView
    
    android:theme="@style/Widget.BottomNavigationView"

/>

 Navigation:
THEMES :

<style name="NavigationView" >
<item name="fontFamily">@font/helvetica_neue_light</item>
</style>

android:theme="@style/NavigationView"