Add translations for `site.navigation.TC39.agenda` / `site.navigation.TC39.notes`

[smorimoto]

Translation status of each language

Anyone interested in working on this, raise your hand on this issue. (To prevent duplication of effort among multiple contributors!)

Language	Status	Link
German Translation	🚧	tc39.github.io/de/site.json Lines 7 to 8 in be8c92e "agenda": "Meeting Agendas", "notes": "Meeting Notes",
French Translation	🚧	tc39.github.io/fr/site.json Lines 7 to 8 in be8c92e "agenda": "Meeting Agendas", "notes": "Meeting Notes",
Japanese Translation	✅	tc39.github.io/ja/site.json Lines 7 to 8 in be8c92e "agenda": "議題", "notes": "議事録",
Russian Translation	✅	tc39.github.io/ru/site.json Lines 7 to 8 in be8c92e "agenda": "Meeting Agendas", "notes": "Meeting Notes",
Simplified Chinese	✅	tc39.github.io/zh-Hans/site.json Lines 7 to 8 in be8c92e "agenda": "会议日程", "notes": "会议记录",

[Jack-Works]

It looks like there are only 2 lines that need to be translated?

[smorimoto]

@Jack-Works Yes

[smorimoto]

Well, there's still one more: #320

[Jack-Works]

btw zh-Hans is Han Simplified, Traditional Chinese should be zh-Hant. I think we can also add zh-Hant if there is a need and it's simple to do so.

[Jack-Works]

Opened #417 for zh-hans