partner-contact/partner_contact_lang/views/res_partner_view.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record id="view_partner_form" model="ir.ui.view">
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form"/>
        <field name="priority">9999</field>
        <field name="arch" type="xml">
            <!-- we have to replace it because it's not inside a group -->
            <xpath expr="//field[@name='child_ids']/form//field[@name='lang']" position="replace"/>
            <xpath expr="//field[@name='child_ids']/form//field[@name='mobile']" position="after">
                <field name="lang"/>
            </xpath>
        </field>
    </record>

</odoo>