A number of people have posted videos on YouTube featuring them speaking various languages. In some cases they speak many different languages in the same video, in others they speak different languages in different videos. Some videos appear to be spontaneous, while others have a more scripted feel to them. There is also perhaps an element of showing off and one-upmanship involved.
Some examples include:
- Burkhard Lee: speaks German, English, French, Mandarin, Thai, Japanese and Indonesian
- Richard Simcott: speaks English, French, Spanish, German, Dutch, Macedonian, Swedish, Italian, Portuguese, Welsh, Catalan, Czech, Serbian/Croatian/Bosnian, Romanian, Esperanto, Icelandic, Russian and has some knowledge of quite a few others
- Steve Kaufmann: speaks English, Cantonese, Mandarin, French, Japanese, Korean, Spanish, German, Italian, Russian, Portuguese and Swedish
- Stuart Jay Raj: speaks over 15 languages, including Thai, Mandarin, Japanese, Hindi, Indonesian and Vietnamese
- Luca the Italian Polyglot: speaks Italian, Portuguese, Swedish, Engilsh, French, German, Dutch and Russian
- Benny the Irish Polyglot: speaks English, Irish, French, German, Spanish, Italian, Portuguese and Esperanto
There are quite a few others and you can find more examples here.
Videos like this, and audio recordings, are one way to demonstrate that you can speak the languages you claim to speak, and your level in each language. They could also be used to demonstrate your progress to yourself and others. You could make one or more every so often and compare them to see improvements in your language abilities.