It is possible, but maybe some of the output views sanitize the output and therefor you lose markup.

But..... markup shouln't be part of a translation. Markup should be done in the views and the translations should only provide the text.

What if somebody forgot to add the html tags in their translation (eg, 'nl') than the page looks different depending on the user language???

You can add any HTML tags. For example:

'blog:header' => '<h2>Blog Header</h2> <a href="'.elgg_get_site_url().'blog">My blog</a>',

Run flush the caches after saving changes.

Thanks guys. 

Sorry I used the wrong example. I wanted to format my notification message in an email body. for example to make it bold for important paragraph.

In the default Elgg notifications all html tags get stripped. You need an e-mail handler that can generate HTML e-mails if you wish to keep the markup

Stipping of html tags: https://github.com/Elgg/Elgg/blob/2.3/engine/lib/notification.php#L641

Look at the HTML email handler plugin for HTML support https://elgg.org/plugins/709492

Thank you very much Jerome. That is exactly I am looking for

@Seri_ng, You might also want to check this one out if you have time   Responsive HTML Email Morden located here   https://elgg.org/plugins/2774609

First, download and install HTML email handler. Second, Follow the usual Elgg module installation practices, I.e. put the directory HTML Email Morden plugin in your /mod directory and unzip it. and enjoy.

Let me know how it goes!

Thanks Tom!!