\selectlanguage command and the
otherlanguage environment have the same functionality: change all language-related settings; the difference is that the environment defines a group so the language change is local to the environment.
In your case, since the changes must apply to whole chapters, I'd probably go with
\selectlanguage, unless intermixing
left-to-right typesetting with right-to-left typesetting is required, in which case the environment would be better according to
otherlanguage does basically the same as
\selectlanguage, except the language change is local to the
environment. This environment is required for intermixing
left-to-right typesetting with right-to-left typesetting.
There's also an
otherlanguage* environment in which the typesetting is done according to the rules of the other language, but the text-strings such as "figure", "table", etc. are left as they were set outside this environment.