app-odoo/app_odoo_customize/static/src/webclient/user_menu.xml

<?xml version="1.0" encoding="UTF-8" ?>
<templates xml:space="preserve">
    <t t-name="app_odoo_customize.UserMenu" t-inherit="web.UserMenu" t-inherit-mode="extension" owl="1">
        <xpath expr="//t[@t-key='element_index']" position="before">
            <t t-if="app_show_lang">
                <t t-foreach="app_lang_list" t-as="lang" t-key="lang.id">
                    <DropdownItem dataset="{ lang: lang.id, code: lang.code }" onSelected="() => this.setLang(lang.code)">
                        <img class="flag"  t-att-src="'/app_odoo_customize/static/src/img/flags/' + lang['code'] + '.png'"/>
                        <t t-out="lang.name"/>
                        <i class="fa fa-check" t-if="user.lang == lang.code"></i>
                    </DropdownItem>
                </t>
            </t>
        </xpath>
    </t>
</templates>