Options->About PKHeX->Shortcuts
Gen1/2 don't store language; PKHeX tries to detect a language if there's any. If there isn't, the combobox's value doesn't change. It's only really there to help you switch a species' language-nickname.
If you click the OT label and have it apply OT details, it'll slap on the savefile's OT trash bytes in addition to the relevant trainer details (gender, geolocation).
To bring up the trash byte editor for a given field (nickname/OT), hold control and click on the textbox.