What's the problem?
Because Tiki is available in 25+ languages, whenever you add a textual element to the User Interface (UI), you need to do this in a way that will support its translation.
Creating a translatable string in a Smarty file
The easiest way to add textual elements to the Tiki UI is of course to modify a Smarty Template File (i.e. a file whose extension is .tpl)). In that case, you can make the string translatable by enclosing it in {tr}{/tr} Smarty block.
For example, if you type this in the .tpl file:
{tr}Hello world{/tr}
Then it will be displayed as "Hello world" for English users, but it will be displayed as "Bonjour le monde" for French speakers (assuming that this translation has been created... more on this later).
Warning: Do not use single quotes around a textual element!
Often, the textual element you are adding is part of some HTML or Java script attribute. For example, you might use text in the ALT tag of an icon as follows:
alt="{tr}Click on icon to start monitoring this page{/tr}"
When you do this, it's important that you use double quotes "" instead of single quotes '' to enclose the translated text. The reason is that in many languages, the apostrophe is the same as the single quote. So if one of the translations includes an apostrophe, it will be taken to be the closing quote for the ALT tag.
Creating a translatable string in a PHP file
Sometimes, UI textual elements must be created in a PHP file (i.e., a file that ends with .php) and then passed on to a .tpl (through a $smarty->assign() call) that will then display it.
In that situation, you must invoke the tra() (or tr()) function on the textual element, before passing it to $smarty->assign().
Placeholders
In many cases, a string will contain variables that need to be included, like a number or a name. Tiki provides a placeholder mechanism to help with this. As much as possible, try to write sentences in a gender/number neutral manner to make translation possible.
Never use multiple translation segments to introduce a variable. Translators won't be able to translate correctly.
$text = tr('There are %0 post(s) in this forum. The last one was by %1', $count, $lastAuthor);
{tr _0=$count _1=$lastAuthor}There are %0 post(s) in this forum. The last one was by %1{/tr}