You should probably learn from a successful example, like Microsoft ResX, the .NET way. It is XML, so its easy to manage with code, or by hand. The XML schema is very simple, not crazy at all.
I'd also avoid editing them by hand (especially if they require encoding) and provide your co-worker a friendly editor for whatever format you decide on, so I'd stick with a standard format like this and reuse one of the editors out there.
.resx is workable, but you can also use the more traditional format, .resource.
A quick Google shows some free resx editors. Lutz (Roeder) has something I would expect to be of decent quality (given he wrote Reflector)
Or this one, looks oriented towards translation end-user
Or use Visual Studio Express (free for your co-worker), or hack one up yourself using the schema (Microsoft ResX Schema v2.0)
The main thing you'll be manipulating will be data elements. You can store text or images or other objects in a resx.
Secondly, I recommend you create less granular resource files, like one per form or screen or page, or even one per app if not that large. It'll be friendlier to manage.