For quite some time when applying formatting in GUI development one either needs to apply directly visual HTML markup or some sort of odd plain text formatting. Thankfully the internationalization system supports more sensible ways of doing this, in particular semantic markup rather than visual markup.
Let me illustrate the difference quickly. Say you have a title in HTML and you want the title to have a special format. The visual way to go about this woud be:
<b><em>This is a Supreme Title</em></b>
the semantic way however looks like:
<h1>This is a Supreme Title</h1>
In a semantics markup you are not defining how something looks, but what it is, the visual appearance of this thing is defined somewhere else. Just like in the example above, if you use the h1 tag you can easily format all primary headers in a unified fashion using CSS, if you use the visual tags and want to change something you will have to manually walk through all your HTML code and change it.
Now, the very same problem applies to strings you might want to use in your code. For instance you might have something like that in your code:
i18n("I deleted <qt><b><em>%1</em></b></qt> muhahaha!", a);
Should you not have a lot to do with i18n or l10n, a translator would get this string in most editors exactly like that (i.e. including your markup). There are multiple reasons why this is bad. For one, the translator could accidentally break the markup (since he needs to copy it into the translated string), for another, the translators do not know the context of this message and will have a hard time translating it properly. Additionally of course the same problem applies as with HTML and changing the appearance.
Applying semantics markup the above string could look like this to provide both context for the translators and useful default formatting for the file name that will be inserted at %1:
i18nc("@info", "I deleted <filename>%1</filename> muhahaha!", a);
If you are developing software using the KDE platform then please checkout the detailed semantics page and use semantics to make our translators’ life easier and your formatting more coherent with the rest of the system.
We at Kubuntu certainly endorse i18n semantics and apply it whenever possible in our software, due to that we created a cheat sheet that helps with remembering the various markups tags .
[Also available as SVG]