ui_locales gives a hint about the desired display language of the login UI
Integrators will need to supply the “ui_locales” parameter when creating the authorization URL that starts the consent flow, as described in the specification here https://openid.net/specs/openid-connect-core-1_0.html#AuthRequest.
To enable Finnish language, the “ui_locales” must have the value “fi-FI”.
Using the ConsoleClientExample code that used is availble GitHub, the first part of the code would look like this:
var client = new OidcClient(OidcClientHelpers.ConfigureOidcClient(RedirectUri));
var extraParameters = new Dictionary<string, string>
var state = await client.PrepareLoginAsync(extraParameters);
The above is simply using IdentityModel, so it also applicable to other implementations in C#.