shouldn't usual be accepted?
That's a valid translation in my opinion.
shouldn't common be accepted?
Common is נפוץ. I don't think they are interchangeable in English either.
Can you also translate רגיל as regular? Although I probably wouldn't say, "this is regular. " (I would say, "this is usual," or "this is normal.")
Depends how you use regular...
?ר.ג.ל How exactly does it relate to the root
That's one of the meanings the root ר-ג-ל has. For example, להתרגל (in binyan Hitpa'el) means 'to get accustomed to-'.
Is it related to the English "regular" or Latin "regulus"?
Hebrew רָגִיל is derived from the noun רֶ֫גֶל foot, from which the root רגל to go by foot and then to go about frequently was created.
OK, thanks. Then, is there any possibility that Latin got it from Hebrew or some other Semitic language?
Well, no regō (to rule, govern) is a very established indogermanic root, meaning originally something like to straighten.
It's not related to English, since it already appears in the Mishnah. My guess that it's not related to Latin either, just an extension of the known Semitic root ר-ג-ל.