Languages change over time and languages carried through migrations and settlement change in new spaces -- and affect the language(s) of those spaces. We can track those changes by using studies of languages, socio-linguistics, as well as such things as dictionaries for primary sources. The Library collections contain dictionaries of most languages from most time periods from around the world. Use the Advanced Keyword search to look for dictionaries (yes, plural) and the language of your choice as a subject. You can further limit by date, if you so choose, or add in a geographical limitation, such as Costa Rica. Sometimes "historical" dictionaries are published later, so you might not want to use the date limitation if you are limiting by country.
The word to search for a dialect is, conveniently, dialects (yes, plural) and the name of the language, e.g., Irish. Some of the material you find may be very oriented towards linguists. If you have too many results, try adding in dictionaries (yes, plural). Again, make sure that you're doing a subject search.
Use the advanced search to look for Webster's Dictionary in the title field and select the time period that interests you.
You can do the same thing using the subject field with Americanisms (aka slang) and Quotations, to get a flavor of the speech of the day.