Hello,
The error occurs because partner_id.lang is not always considered a valid language code at the mail template rendering stage, and sometimes Odoo validation fails when accessing nested fields this way inside <field name="lang">.

In Odoo 19, language codes must follow the strict ISO format ll_CC.

If your module from Odoo 18 includes old or custom codes like en_UK, en_in, fr_BE@euro, or any non-standard code, Odoo 19 will raise a Language Code Error.

Fix:

• Update all language codes in XML, demo data, and PO files to valid formats (e.g., en_GB, en_US, sw_KE, hi_IN).

• Rename PO translation files accordingly.

• Remove or clean invalid res.lang entries.

Once all codes follow xx_YY format, the module will install correctly in Odoo 19.

Hi,

That error usually means that the language data for your database (in res.lang) doesn’t match what v19 expects — Odoo prohibits changing language codes on upgrade via res.lang.csv, so the import fails. 

To solve this:

• Don’t attempt to modify existing language codes when migrating. Keep the original code values in res.lang.

• If you need a new language, add it after the upgrade via Settings → Languages.

• If the module provides custom translations, ensure their .po / .csv files use valid, consistent language codes recognized by Odoo.

Hope it helps.