Work on a project
The home page shows you an overview of all projects you are currently working on. Select the one you want to use by clicking the fat green floating action button with the translate symbol.
This will open the translation page:
The translation page show a list of messages on the left side and a translation editor for the currently selected message.
By default the list shows you all currently untranslated messages. You can switch to all messages (shown in the screenshot above) or you can filter by a specific substring to show only those messages that contain the substring (project in the example):
The right side of the translation page shows the currently selected message.
You see the ID, a hint about the source of the message, and the original message.
In the black framed box you see the translation of the message. You can edit it here. When finished you can change to the next message (or to the previous one). This will accept your changes as new translation.
You can also used the mark as translated button to accept the changes.
The simplest case of a message to translate is just a small piece of text. But there can be some structure. The text can contain placeholders, variable parts that will be filled at runtime with some values. The text can contain some formatting like bold face or italic.
All these cases are handled by some special XML markup in the message. The exact markup depends on the format of the translation file. So XLIFF 1.2 uses a special markup, XLIFF 2.0 uses another totally different one, and XMB/XTB uses a third one.