Composite Layout allows your nodes to be displayed in complex layouts. Currently, two and three column layouts are provided. You can also add other nodes, blocks, and even CCK fields to your node's layout. The content area of your node is divided into zones and you decide what should appear in those zones. Zones are essentially the same as Drupal blocks, but they apply to nodes rather than the entire site.
You can think of Composite Layout as Drupal blocks for nodes.
There is overlap in functionality between Composite Layout and Panels. Both address the issue of complex layouts, but each has a different approach.
Here is an informal comparison:
Panels is more powerful and flexible (I think, I'm not a Panels expert).
Composite Layout is simpler (I hope).
The user interface is different. You may prefer one or the other.
Composite Layout applies on content types, so it can be turned on for any node. Furthermore, you can have more than one composite layout node type.
If you use the Content Construction Kit, Composite Layout allows you to manage the layout of your CCK fields.
Otherwise, it will probably come down to personal preference as to which is more suitable.
Translation helpers enables other modules to respond to changes in the "source translation" of a set of translated content. This functionality is useful for modules that track data by the "source translation" (node.tnid value).
This module doesn't do anything on its own. You should install it only if it's suggested or required by another module you're using.
The module also provides other methods for modules to use with translated content.
Drupal 8
A Drupal 8 version won't be needed. Translation Helpers addresses use cases related to the model of translation used in the Drupal 7 core Content Translation module, in which each language has its own node. Drupal 8 has switched to fields-based translation, in which the awkward workarounds of D7 will be a thing of the past.