The main purpose of the sign gifts were to confirm Jesus as Messiah, and that the Gospel was true, butthat was just during the transisition period recorded in Acts.This really isn't that hard to reconcile. In Acts 2, an interpreter was not needed since they were speaking the languages of those nations, so of course they understood. In 1 Corinthians, it would seem the foreign language was not known in the congregation, therefore it required interpretation in order to be profitable to them. Paul likely spoke tongues in both contexts, in missionary encounters with those who did not speak his language, and in a corporate worship context when there was an interpreter.
Regarding your earlier question of how the gift of tongues confirmed the apostolic testimony, it was not necessarily the message of any particular tongue that confirmed the testimony, but the new presence of the gift itself (again Hebrew 2:3-4). It was a new sign (along with many others) accompanying and confirming the new revelation of the gospel. But it still had to be used according to God's purpose, not for various recipients to show off.