Well, depends. I mean, will people use your UTAUloid in said languages? If not... Will you use? If you answered "no" for both answers, I think it might end up as unnecessary. I've seen people using their own UTAUloids and others' UTAUloids for English, Spanish and French, but I've never seen anyone trying to make them sing in Czech, for example (and there are many interesting Czech songs out there). Thing is... If you're going to record your UTAUloid in these languages and won't even give a try on using it by yourself, people might probably think it's not worth using.
A tip that I kinda learnt with Yesi and Day (without them knowing) and I can give to you (and all UTAU users): If you're going to make a multilingual UTAUloid, make language packs (you'll understand the logic). Start with the language that shares most phonemes with Japanese. Then, you progress on the packs, by the order they share phonemes with the languages first released (and tell which packs are necessary to complete the set). In the end, you'll have various small VBs that people can download once each instead of a huge VB that screws with their connection.
Let's take Romance languages as an example.
If you started with a Japanese VB:
Galician/Portuguese/Romanian (basically same set of phonemes)
Please note that I haven't included Brazilian/African Portuguese nor Guianean/Quebecquois/African French because I didn't notice differences so striking as the ones between European and Latinamerican Spanish.
I dunno the order of the others for sure, but I'm almost sure it's really Spanish nearest Japanese in phoneme sharing and French on the farther end (not that it doesn't share phonemes with Japanese, but it's the one that differs the most).
Of course, you'll have to make sure you know how to pronounce all the Slavic languages really well. I, for example, speak Brazilian Portuguese, I can pronounce perfectly European and African Portuguese (the difference isn't that big, and no, didn't try Galician nor Romanian), I can pronounce a passable Latinamerican Spanish and an almost cringeworthy Italian, but I completely and utterly suck at pronouncing French.