I see what you mean. One of the follow-on screens did show up in English.
Usually if you use the Chrome browser you'll get a pop-up, asking whether you want an English translation. When I'm using MS Edge, I right-click and then click on "Translate to English". I'm pasting below the info I got that way:
OPENING
HOURS Morning
from 9.00 to 13.00
Afternoon
from 14.30 to 17.00
Wednesday closed
Admission is allowed up to 30 minutes before closing
For information call (+39) 075.8051419 or write to [email protected]
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A separate page for Museum Reservations (https://www.porziuncola.org/prenotazioni-museo-porziuncola_eng.html) says this:
We look forward to welcoming you to the museum. You may find this information useful during your visit:
Please, arrive on time and let us know if you are late by calling the number (+39) 075.8051419
It is possible to cancel your reservation up to 48 hours before your arrival
The shrine is a sacred place. Please, show respect and decorum, maintain the silence, and heed all recommendations on information signs.
For the schedule of celebrations and further information, visit the website: www.porziuncola.org
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The museum webpage then has a form to fill out that asks, among other things, for the name of the "Group Organizer", so I wonder whether they expect individual travelers just to show up. Like you, I'd be a bit concerned about doing that around the time of the Feast of St. Francis.
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There's a similar reservation page for the Porziuncola, again asking for the name of the "Group Organizer": https://www.porziuncola.org/reservations.html
I can't find any indication that a tour is mandatory. I happen to have a copy of the 2021 edition of Lonely Planet's Italy guidebook, and it doesn't say anything about mandatory tours, either. Perhaps it would be safest to write to the email address given above for the museum ([email protected]) to ask whether walk-up visitors will be admitted during the time you plan to be in Assisi.