Joel of Far Outliers, who posts excerpts from the books he reads, has an entry on a problem I hadn’t really thought about, the meaning of synonym in a situation where there are many distinct dialects. From his translation of a section in Probleme de sinonimie, by Onufrie Vinţeler:
Sever Pop (cf. 1929) used to note that, within the territory of Romania, the following terms can be found to denote the concept of ‘horse trader’: barâşnic, craşcadău, cupeţ, factor, fleşer, geambaş, gheşeftar, ghiambabău, gârgez, făznar, hendler, herghelier, hâmbluitor, liverant, mecler, năstrăpaş, negustor, peţer, pilar, potlogar, precupeţ, precupitor, semsar, sfârnar, sfârnăroiu, şmecher, ţânzar, ţigan, tuşer. No one doubts that all the terms listed denote the same concept. The question that arises is the following: can each and every one of these words be considered synonyms? According to some definitions, still in circulation, all words that express the same notion are considered synonyms. Glancing over the list of words above, we observe that only the word negustor, which is the general term, and to a certain degree the word geambaş, are more widely known and can be considered synonyms; the rest are known only in more or less restricted areas. For the great majority of Romanians, words like barâşnic, gârgez, hendler, mecler, tuşer, and so forth do not mean anything; they are just as unintelligible as any others in a foreign language. Of course, in many places negustor can be a synonym of făznar, and geambaş with herghelier [‘herder’], and so on, but this only happens in certain places and not across the whole territory where Romanian is spoken.
These examples prove once again that for two or more words to be considered synonyms it is not sufficient that they express the same notion. And in cases of regional synonymy, the notion of synonym must be localized and made concrete.
There’s further discussion of words for ‘corn, maize’; the “standard” word porumb (which, oddly, used to mean ‘pigeon, dove’; the latter meaning is now expressed by porumbel) has several widely known equivalents, as well as (in one region) the loan word tenchi (borrowed from Hungarian tengeri, itself a synonym in Hungarian of kukorica, which is presumably from Slavic). It certainly makes sense to consider as synonyms only words that are in competition with each other in the same dialect; I can’t go along with Joel, who doesn’t “have any problem with considering terms in different languages to be synonyms”—that seems to me to stretch the sense of synonym beyond the bounds of usefulness.