I believe your suggestion is not enough at the moment.
I already cloned the "line items table" module and translate every possible fields, but it's not possible to translate the billing frequency of a single item. In the module I see a variable {{ item.recurringbillingfrequency }} and in my quotes I always see these lines in English.
Also, for Comments and Terms I don't find a way to translate the label or clone the modules as they seem to be "hardcoded".
I tried to change the labels in Settings > Object > Quotes but if I do so, all my quotes show the default text (Lorem ipsum dolor...) instead of my custom box text.
Right now the Custom Quote Template functionality does not have a multi-lingual aspect to it yet. We're aware of this need and are working towards it but it may be a little bit. In the mean time you can clone the modules we provide: @hubspot/cms-quotes-theme/modules/line_items_table.module
And where there is hardcoded english text, you can add fields to make it so the text is editable.
The advantage here is that you can then create 1 coded template, which can be used for different "sales manager templates" (one for each language you want to support).
In the future this workaround shouldn't really be needed or a different workflow will exist for handling separate languages. Making these changes now however wouldn't "hurt" the future workflow.