How to Define Translations in Custom Modules & Add Languages in Odoo 18

from odoo import models, fields, api, _
from odoo.exceptions import ValidationError

class InventoryRequest(models.Model):
    _name = 'inventory.request'
    _description = _('Inventory Request')  # Translatable
    
    name = fields.Char(string=_('Request Number'), required=True)
    state = fields.Selection([
        ('draft', _('Draft')),           # Translatable selection
        ('pending', _('Pending Approval')),
        ('approved', _('Approved')),
        ('rejected', _('Rejected')),
    ], default='draft', string=_('Status'))
    
    notes = fields.Text(string=_('Internal Notes'))
    
    def action_submit(self):
        """Submit request for approval"""
        if not self.product_lines:
            raise ValidationError(_('Please add at least one product line.'))
        self.state = 'pending'
        return {
            'type': 'ir.actions.client',
            'tag': 'display_notification',
            'params': {
                'title': _('Request Submitted'),  # Translatable notification
                'message': _('Your request has been submitted for approval.'),
                'type': 'success',
            }
        }

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <record id="view_inventory_request_form" model="ir.ui.view">
        <field name="name">inventory.request.form</field>
        <field name="model">inventory.request</field>
        <field name="arch" type="xml">
            <form>
                <header>
                    <!-- Button labels auto-translate via string attribute -->
                    <button name="action_submit" type="object" 
                            string="Submit for Approval" class="btn-primary"
                            invisible="state != 'draft'"/>
                </header>
                <sheet>
                    <!-- Group labels translate via string attribute -->
                    <group string="Request Details">
                        <field name="name"/>
                        <field name="state"/>
                    </group>
                    <!-- Labels in notebook pages -->
                    <notebook>
                        <page string="Products" name="products">
                            <field name="product_lines"/>
                        </page>
                        <page string="Notes" name="notes">
                            <field name="notes" placeholder="Add internal notes..."/>
                        </page>
                    </notebook>
                </sheet>
            </form>
        </field>
    </record>
</odoo>

inventory_request/
├── models/
│   └── inventory_request.py
├── views/
│   └── inventory_request_views.xml
├── i18n/
│   ├── fr.po          # French translations
│   ├── de.po          # German translations
│   └── es.po          # Spanish translations
├── __init__.py
└── __manifest__.py

# Translation of Odoo Server.
# This file contains the translation of the following modules:
# inventory_request
#
msgid ""
msgstr ""
"Content-Type: text/plain; charset=UTF-8\n"
"Language: fr\n"

#. module: inventory_request
#: model:ir.model,name:inventory_request.model_inventory_request
msgid "Inventory Request"
msgstr "Demande d'inventaire"

#. module: inventory_request
#: model:ir.model.fields,field_description:inventory_request.field_inventory_request__name
msgid "Request Number"
msgstr "Numéro de demande"

#. module: inventory_request
#: model:ir.model.fields.selection,name:inventory_request.selection__inventory_request__state__draft
msgid "Draft"
msgstr "Brouillon"

#. module: inventory_request
#: model:ir.model.fields.selection,name:inventory_request.selection__inventory_request__state__pending
msgid "Pending Approval"
msgstr "En attente d'approbation"

#. module: inventory_request
#: code:addons/inventory_request/models/inventory_request.py:0
msgid "Please add at least one product line."
msgstr "Veuillez ajouter au moins une ligne de produit."

#. module: inventory_request
#: code:addons/inventory_request/models/inventory_request.py:0
msgid "Request Submitted"
msgstr "Demande soumise"

"id","name","code","iso_code","direction","grouping","decimal_point","thousands_sep","date_format","time_format","week_start"
"base.lang_sw","Swahili / Kiswahili","sw","sw","ltr","[]",".",",","%d/%m/%Y","%H:%M:%S","1"

{
    'name': 'Inventory Request',
    'version': '18.0.1.0.0',
    'depends': ['base', 'stock'],
    'data': [
        'security/ir.model.access.csv',
        'data/res.lang.csv',  # Add new language
        'views/inventory_request_views.xml',
    ],
    'installable': True,
    'license': 'LGPL-3',
}