Find a particular IETF language tag?
If you know the name of the language you're after, but not the IETF language tag assigned to it, visit the Subtag registry. Search on the name of the language, and you should find a record with type "Language". You can then also narrow down to a region where the language is spoken: for instance, search on "united kingdom" and you'll get a record with type "Region" and a code (bizarrely) of "GB". Put a hyphen between them. Thus, English in the UK has the code "en-GB"
Also, your browser may already know the language code you use. Visit this page and look for the Accept-Language header value. If you bought your device in the same region as the language you speak (for example, you speak Canadian English and you bought your computer in Canada), then your code may already be present in the header.