Like so much of early (La)TeX software, BibTeX's assumptions were firmly rooted in what its author knew well, viz., academic papers in English (particularly those with a mathematical bent). BibTeX's standard styles all address exactly that problem, leaving the user who writes in another language (or who deal with citations in the style of other disciplines than maths) to strike out into contributed software.
For the user whose language is not English, there are several alternatives. Possibly most straightforward is to switch to using biblatex, which can produce a bibliography appropriate to several languages. However, biblatex is large and has correspondingly large documentation (though it is well-written and pleasingly typeset), so its adoption takes time.
Otherwise, the simplest procedure is to provide translations of BibTeX styles into the required language: the solitary finplain.bst does that for Finnish; others one can find are for Danish (dk-bib), French (bib-fr), German (germbib), Norwegian (norbib) and Swedish (swebib) bundles (of which the bib-fr set is the most extensive). The spain style implements a traditional Spanish citation style.
These static approaches solve the problem, for the languages that have been covered by them. Unfortunately, with such an approach, a lot of work is needed for every language involved. Two routes to a solution of the “general” problem are available - that offered by babelbib, and the `custom-bib` mechanism for generating styles.
Babelbib (which is a development of the ideas of the bibgerm package) co-operates with babel to control the language of presentation of citations (potentially at the level of individual items). The package has a built-in set of languages it “knows about”, but the documentation includes instructions on defining commands for other languages. Babelbib comes with its own set of bibliography styles, which could be a restriction if there wasn't also a link from custom-bib.
The makebst menu of custom-bib allows you to choose a language for the BibTeX style you're generating (there are 14 languages to choose; it looks as if spain.bst, mentioned above, was generated this way). If, however, you opt not to specify a language, you are asked whether you want the style to interact with babelbib; if you do so, you're getting the best of both worlds - formatting freedom from custom-bib and linguistic freedom via the extensibility of babelbib