Support two languages simultaneously

(copy-pasted from a post comment) The Sony Android soft keyboard has an amazing feature: it lets you have more than one language active at once. When you start writing you will get suggestions from both languages, but within a few words it will settle on the language you currently use (though still give you exact matches in the other). If you start typing in the other language it will again shift to that language by the third word or so.

This is extraordinary useful. It's a constant irritant to have to manually switch languages all the time, and this completely eliminates it.

Example:

* The user has Swedish and English enabled. She starts writing "s t o r", and the suggestions "stor" (Swedish for "big"), "storage" and "storm" ("storm" in both Swedish and English) appear. She selects "storm". That's valid for both languages, so no decision is made.
* Next she writes "eye" which exists only in English. The input notes that but a single word is too little to go on.
* "is" is a word in both languages, so it doesn't change the language determination.
* "very" is again English, and now we have two recent votes for English and none for Swedish, enough to decide.
* She starts typing "l a r", and English "large" comes up as a suggestion, while Swedish "larvig" (silly) does not.
* She continues to type and actually writes "larvig". Since it's an exact match, the input will show it even though it uses English as the current language.

In an ideal world, this would cover Japanese, Chinese and other non-alphabetic languages as well. They have long used the "present alternatives while the user types" approach already, and at least for Japanese it is common to spell out the word in alphabet, and get kana and kanji suggestions (I'm sorry I don't know the typical way for Chinese, Thai, Korean/Hangul or other languages).

So one approach for Japanese would be to present the hiragana transliteration as the sole Japanse option. If the user selects that, you'd switch to Japanese mode where all suggestions would be japanese except for the romaji (alphabet) transkription. Selecting that would again switch back to alphabet mode.