Add lang to the Contract model and tree view
CHANGELOG:
### Added
- [#735](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/735) Add Partner lang to Contract model and show it in the Contracts tree view.
Related card: https://trello.com/c/gVlFHbHr/1152-idioma-env%C3%ADo-correos
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![image](/uploads/8f4e327ce76195a3a04311a23fcaa826/image.png)
Edited by Pelayo García