For now I've found a workaround for this problem but I am still interested in info on including the tags in language files. The workaround I am now using is:
In order_invoice.tpl:
PHP Code:
<br />{$lng.eml_invoice_bank_transfer} <font color="#C00000"><b>{$lng.lbl_order_id} #{$order.orderid}</b></font><br /><br />
And then in eml_invoice_bank_transfer I first start with our bank details and end with:
PHP Code:
<font color="#C00000"><b>When effecting payment please mention:</b></font>
Which generates:
When effecting payment please mention: Order id #123