It depends on your level of experience I think. If you are new to programming you'd probably wish to stick with one language and learn it well before moving on to another one. Otherwise you might get basic concepts confused.
Afterwards though I see no problem in using multiple languages at the same time. Most large projects will tend to include bits in various languages so you can't really avoid it anyway.
I also don't draw much of a distinction between using and learning a language. Beyond the basic syntax, and some speical constructus, you'll need to continually learn the API and best practices of whatever language you are using.
The more often you encounter a language that more you should follow-up on the syntax and look for language-speicfic approaches to solve your problems. It'd be bad to try and force the same syntax, or approach, into all the languages you use.
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