| Part of speech | unknown part of speech |
|---|
| Singular | Plural | |
|---|---|---|
| Nominative | diftongo | diftongoj |
| Accusative | diftongon | diftongojn |
| Catalan | diftong |
|---|---|
| Dutch | diftong; tweeklank |
| English | diphthong |
| German | Diphthong; Doppellaut; Zwielaut |
| Hungarian | diftongus |
| Low German | diftong |
| Papiamento | diptongo |
| Portuguese | ditongo |
| Spanish | diptongo |
| Swedish | tveljud |
| Welsh | deusain; dipton |