partner-contact/partner_contact_lang/views/res_partner_view.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <!-- Copyright 2020 Pesol <pedro.gonzalez@pesol.es>
     License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl). -->
    <record id="view_partner_form" model="ir.ui.view">
        <field name="name">view.partner.address.form.extension</field>
        <field name="model">res.partner</field>
        <field name="priority">100</field>
        <field name="inherit_id" ref="base.view_partner_form" />
        <field name="arch" type="xml">
            <xpath
                expr="//field[@name='child_ids']/form/sheet/field[@name='lang']"
                position="replace"
            >
                <field name="active_lang_count" invisible="1" />
                <group attrs="{'invisible': [('active_lang_count', '&lt;=', 1)]}">
                    <group>
                        <field name="lang" />
                    </group>
                </group>
            </xpath>
        </field>
    </record>
    <record id="view_res_partner_filter" model="ir.ui.view">
        <field name="name">res.partner.filter.extension</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_res_partner_filter" />
        <field name="arch" type="xml">
            <xpath expr="//field[@name='parent_id']" position="after">
                <field name="lang" />
            </xpath>
            <xpath expr="//group[@name='group_by']" position="inside">
                <filter
                    name="language"
                    string="Language"
                    context="{'group_by' : 'lang'}"
                />
            </xpath>
        </field>
    </record>
</odoo>