With the latest update, MLZ now has enhanced formatting support for supplementary data:
The hard-wired [square braces] that formerly set off the secondary and tertiary versions of multilingual fields have been replaced by per-field affixes, configurable via a pull-down menu when fields are selected as shown in the screenshot to the right. Clicking a second time without choosing a selection will set the supplementary field off with a single space as affix. Supplementary content will now appear outside of any quotes enclosing the primary content, and bold, oblique or italic font styling is suppressed on the supplementary content.
There is room for more flexibility in this design; if you have use cases that this layout does not cover, let me know.