It's easy! See Install Team Calendars.

There are a few reasons:

• We feel we can solve a lot of specific problems with the combination of events, people, projects (JIRA applications) and content. Atlassian sees this plugin working alongside your personal meeting or events calendar.
• Not every customer needs a team calendaring solution. Hence, by not bundling Team Calendars with Confluence, it doesn't increase the cost of Confluence for those customers who aren't going to use it.
• The old plugin was not maintained or supported. In contrast, Team Calendars is maintained and supported moving forward.

The old calendar plugin was never officially supported and as such carries no support from Atlassian. It is currently available as an open-sourced component, available on plugins.atlassian.com for use with legacy versions of Confluence (3.4 and earlier). The old calendar plugin does not work with Confluence 3.5.x or later versions. There are currently no plans to continue development on the old calendar plugin. If you are currently using the old plugin, there is a manual migration path to Team Calendars available by importing your iCal files.

To give the Team Calendars team feedback, raise an issue in the TEAMCAL project on jira.atlassian.com.

はい、できます。詳細はこちらを参照してください。

• Google カレンダーから Team Calendars を登録する
• Confluence から Google カレンダーを登録する

This usually indicates that something is wrong with your license. Check that it is properly installed by checking 'Confluence Admin' > 'Team Calendars License'. Also, log in to https://my.atlassian.com and check the status of your license.

Yes. All you need is a Team Calendars license for Confluence Data Center. Learn more about Data Center licensing

Yes you can! This is now possible and was released as part of Team Calendars 2. See Add JIRA events to find out more.

Although this is not officially supported, it does work on the iPhone. All you have to do is select "subscribe" from a specific calendar menu and login using your Confluence details. iOS should automatically attempt to add the calendar to your iPhone. More instructions here: Subscribe to Team Calendars from Apple iOS Calendar.

はい、できます。ユーザー フィールドに複数の名前を追加します。

As of Team Calendars 1.1, the plugin is available in English and the bundled Confluence languages (French and German). We hope to have this plugin also available on translations.atlassian.com for other language support. You can stay updated on this issue and provide feedback on this issue.

日付形式は言語設定に依存し、それに応じて自動的に調整されます。これは Confluence のユーザーおよびグローバル設定です。管理者はサイトの既定言語を設定でき (「既定言語の選択」を参照)、ユーザーは自身のユーザー設定を設定できます (「ユーザー設定の編集」を参照)。

The Team Calendars follows per user language preferences to decide which day is at the start of the week. For example, if you want to make the start of the week to Sunday (US Calendar), you would need to install the US Language pack and edit your Confluence user preferences to choose US English. To do this, click User name > Settings > Edit > Language and then choose US English. 
 The Confluence default is to Automatically detect browser settings.

Currently, it's not possible to subscribe to Google Public Calendar from Team Calendars. This is a known issue with Google Calendar; it appears that Google Calendar Public .ics file doesn't contain the data/event that can be read by Team Calendars to display events. Only Google Private Calendar can be subscribed from Team Calendars for now.