You are right, I mixed them up (that comes from not checking).
The usecase for monolingual text are a bit rare, and I am thinking of things like official motto (which is usually not translated),
I think if it is only "usually not", but sometimes indeed translated, using multilingual for the property would be a better choice. If only one language is available, the language fallback would always end with this.
etymological annotations, or the official name of a company (also, usually not translated),
"usually". Companies sometimes do run under local names (or variations): de: Sanyo Denki K.K. ja: San’yō Denki Kabushiki-gaisha, en: SANYO Electric Co. Ltd.
carefully about how delineate them from each other in the entry forms, or otherwise it might end up a bit messy.
I think when adding the option for non-linguistic content (= ISO zxx) for language-neutral entities (e.g. for scientific species names, post codes), this type is the least needed.
(If anything, it may be more valuable to add a default flag to indicate a primary name that should be used prior to the first in a language fallback. This would be valuable in "mixed" cases, where a string is translated in a few cases, but not in the majority of languages (the "usually not case"). Else in a rare border case, where a German company that provides translations to Japanese and Chinese, but not to english, a language fallback chain that does contain German may accidentally end up with Chinese. This solves a border case within the multilingual type which I believe cannot be solved with monolingual text.)
Gregor
(Often wrong but never in doubt :-) )