How do I decide what supplemental information should be uploaded for Harvard application?

I am having a hard time deciding what supplemental information should be uploaded to Harvard? (1) I learned some difficult languages on my own - should I submit writing samples to prove it? (2) I studied a language in elementary school - should I submit writing samples and report cards? (3) I have a nice creative writing class essay - should I submit it? (4) My teacher (also a recommender) wrote me a great email on some work I was doing and how I inspire her - should I submit to Harvard? (5) Should I take a photo of my awards and certificates and send? I am confused - I don’t want to overdo it but at the same time want to share these with Harvard.

When applying to one of the most competitive colleges in the country, you should aim to know enough about them to understand what might move the needle, and what’s just fluff. Or desperation. Or not thinking.

Why are you asking about lower school, some class assignment or a private note from a teacher? Do you have an idea what matters to them vs what’s going to make them scratch their heads?

You wouldn’t submit those things with an NHS app, no idea if you’re serious here.

Not sure what you just conveyed… NHS app - National Honor Society? Am I being serious - of course.

There’s an old adage - the thicker the file, the thicker the kid. An AO will spend about 10 minutes reading your application. More stuff does not equal more time.It just means other things will be glossed over. Everything you listed is fluff.

Here is what Harvard says:

https://college.harvard.edu/guides/prospective-students/application-tips

None of those are examples of the kind of information that should be submitted as a supplement to Harvard - or any peer schools.

Examples of things that MIGHT be considered for submission:

An abstract of a research paper (Published in a peer reviewed journal or semi-finalist or better in a national or international competition.) Abstract only! If Harvard admissions is interested they will contact you for the full paper, and possibly request a supplemental recommendation from your mentor.

Audio or visual supplements of noteworthy performances, like the time you soloed with the New York Philharmonic or debuted at the San Fransisco Ballet (both examples of submissions from actual, accepted H students) - not your 10th grade glee club Christmas Gala.

In other words: brief supplemental submissions of extremely noteworthy (on a national or international level) work might be warranted.

On the common app, there is a section about the language you know, I think that is a good place to show the language abilities you have achieved. Similarly, you should have put in the awards/achievements you got in your HS in the common app school activities/EC sections.
Re the great email your teacher sent to you, since she is writing your LOR, I think you got it covered as well.
Show your ability in writing in the essays you have to submit for CA and Harvard.
Good luck.