Android – How to change bottom navigation bar text’s font family

add the font file in the res/font/ folder to bundle fonts as resources

then

You can change it using style resources.
In your styles.xml:

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

Then apply it as a theme in your view:

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

Just checked on my app, it works fine.

reference:
https://developer.android.com/guide/topics/ui/look-and-feel/fonts-in-xml.html#fonts-in-code