It is like I tried to say, in the Bible, there IS NO PRAYER LANGUAGE except your own spoken tongue, or in the case of the 1st Century Church, Genuine spoken languages which were miraculous tongues, but they were still genuine languages.
Since the obvious nature of tongues being genuine languages, then the verse you just cited must be reasoned with that in mind.
There is no clear teaching in that verse that the language spoken by that man in this verse is a foreign never to be understood language, only a language that no one was there to translate, and that would therefore only be understood by God.
Since Angels are spiritual beings, whatever they speak might be called a spiritual language, but we are not spiritual beings are we? We are fleshly beings, and we would therefore speak fleshly languages. "That which is of the Spirit is Spirit, and that which is of the Flesh is Flesh" (John 3)
My point is that tongues are NOT a sign for believers, and the modern Charismatic movement claims it is a SIGN FOR BELIEVERS to prove they have received the Spirit.
The Bible says it is a sign for UNVBELIEVERS.
To claim something is for Believers when the Bible says it is for UNBELIEVERS, that is totally unbiblical.
Yes, it is too much to expect tongues as proof of salvation. But it is also too much to expect they cannot be used without being a sign at all. The passage only means that WHEN they are a sign, THEN they are such to unbelievers (not unvbelievers).