"Eсли бы я знал как" sounds like a poor translation from English. "To know how <to do something>" is better translated as "уметь." Of course, there may be situations when it is alright to say "Я знаю, как это сделать", but in most cases "уметь" works better.
To me, it IS a poor translation. "Если бы я только знал, как говорить на твоём языке" is not idiomatic Russian.No, it is not a poor translation. Quite possibly, a calque from the past, but a valid one.
Of course, but "If I only knew <something>" is not the same thing as "If I only knew how <to do something>"."Если бы я только знал" is indeed "If only I knew".
And again I don't agree. What about such phrases as "я могу говорить об этом часами", "я могу размышлять над чем-то часами"? These expressions are quite common, aren't they?"Говорить часами" usually has a negative connotation, i.e. means "talk and talk uselessly". This is not the case, so the phrase is out of place here.