Hi Mihai,
If the filter is set to use non-HTML syntax with {mlang xx}..{mlang} constructions you can insert these constructions with WYSIWYG text editor. But you must be careful not to insert any line or paragraph breaks between these construction.
If you type in a paragraph:
{mlang en}English text{mlang}{mlang fr}Texts in French{mlang}
this will work. But if you break this in separate lines or paragraph like this:
{mlang en}English text{mlang}
{mlang fr}Text in French{mlang}
this will not work, because between ending {mlang} and {mlang fr} the editor will insert a <br> tag or a pair of </p><p> tags and this will break the syntax of the multy-lingual block.
This syntax is not tested enough. It must be observed more in which cases the filter works well and in which cases it does not work and some rules must be formulated to follow so to make the filter to work.