Create a custom module that inherits the xml from the form you want to modify. Here is an example of the code you should use.

<record model="ir.ui.view" id="view_partner_form2">
    <field name="name">res.partner.form.inherit2</field>
    <field name="model">res.partner</field>
    <field name="inherit_id" ref="base.view_partner_form"/>
    <field name="arch" type="xml">
        <field name="lang" position="replace"/>
    </field>
</record>