Update: HTC has contacted Jeff directly and corrected themselves. Evidently, it was due to "lack of time" as to why the extra language packs were not included and that they will be added in an update. As interesting, they said "by February this should not be an issue". which gives more credence to that rumored update coming down the road.
It turns out that the choices for keyboard languages (ergo configuration, see Settings --> Keyboard) are an option left up to OEMs and carriers. While most Windows Phones have multiple languages supported (e.g. French, German, Italian, Spanish and English) the HD7 curiously has only English on board.
Why that is remains a mystery, but at least according to HTC, the decision was not theirs:
If your Windows Phone 7 does not contain a display language that you would like to use, there is currently no way to get a new language pack installed on the phone. Microsoft makes language packs available (as they're created) to the device manufacturers (OEMs) at the time of manufacturing and it is up to the device manufacturer and mobile operator to decide which languages to include. In this case, it was T-Mobile’s decision to only include English.
As a result, some Spanish speaking Americans (who make up more than 12% of the population) are a little bit upset. Specifically, when they type in Spanish, it auto-corrects their words since the dictionary is dependent upon language selection. What's more, the HD7 on T-Mobile seems to be alone in this single language support when compared other WP7 devices (as far as we know at the moment).
No response from T-Mobile on the issue, but hopefully they'll make a note of it to include in an update in the future. Till then, you multi-lingual folk may want to steer clear of that phone if you think this matters to you.
[See also possible reception and magenta camera issues]
Source: T-Mobile Support Forums; Thanks, Jeff G., for the link