¿Cómo se dice en español?
When did you first go overseas?
Since when have you been working here?　
Since when？　

When was the first time you went to Japan? 　
When did you go there last time?
When is the last time that you went on a trip overseas?

Question

¿Cómo se dice en español?
When did you first go overseas?
Since when have you been working here?　
Since when？

When was the first time you went to Japan? 　
When did you go there last time? 
When is the last time that you went on a trip overseas?

Answer

When did you first go overseas? Cuándo fue la primera vez que fuiste al extranjero?
Since when have you been working here?　Desde cuándo llevas trabajando aquí?
Since when？　Desde cuándo?

When was the first time you went to Japan? 　Cuándo fue la primera vez que fuiste a Japón?
When did you go there last time? Cuándo fuiste allí por última vez?
When is the last time that you went on a trip overseas? Cuándo ha sido la última vez que fuiste de viaje al extranjero?

Aquí las tienes traducidas =)

Saludoss

Answer

Sergio did it great, I'd just add another option for

Since when have you been working here? -  ¿Desde cuándo trabajas aquí?