Japanese version of AMP Fest 2020 event page contains untranslated English text

### Description

While browsing the Japanese version of the AMP Fest 2020 event page, I noticed that a large portion of the content is still displayed in English even though the language is set to JA. The language switcher updates the URL and navigation correctly, but the main headings and page text don’t change.

### Reproduction Steps

### URL
https://amp.dev/ja/events/amp-fest-2020

### Steps to Reproduce
1. Open the page above
2. Ensure language is set to JA (Japanese)
3. Observe page content

### Expected Behavior
All visible text should appear in Japanese when JA locale is selected.

### Actual Behavior
Large portions of text (e.g., "Rewatch the talks", event descriptions, section headings)
remain in English even on the Japanese version.

### Tested Environments
- Browser: Brave (stable)
- Also tested in Incognito mode — same result
- OS: Linux

### Relevant Logs

```shell

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### Browser(s) Affected

_No response_

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start at https://amp.dev/ja/events/amp-fest-2020 and inspect how the JA locale supplies the event page content, including the headings, descriptions, and “Rewatch the talks” text. Done means the visible AMP Fest 2020 content is Japanese when the JA locale is selected, while navigation and the language switcher continue to work.

Written by the indexing model from the issue text.