1. That's difficult to pinpoint. But yes, if the believer wants others to understand what they're saying, i.e., convert them, then it's their onus to have their language interpreted for them. That's why - I think - Christians have been routinely persecuted throughout history. They weren't doing a good job at it. I'm oversimplifying, of course. But, once again, I'm treading between "a translation isn't possible, they're speaking gibberish" and "no one's doing a good job translating." I'm more inclined towards the latter.
2. Many words and concepts aren't immediately translatable between languages. Though the Christian language may seem like "glossolalia" to non-Christians, we may, with good reasons, assume that the language makes sense to them. For instance, when in Japanese, ただいま (tadaima), literally translated to "just now," is used to mean "I'm back home." To those of who aren't Japanese, "just now" makes absolutely no sense. But we don't immediately say the Japanese are speaking gibberish. We say we're not part of the Japanese culture to understand this phrase. Could we also not adopt the same attitude towards Christianity when they talk about "the Holy Spirit" and "Grace?"