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Adaptive spell checking for multilingual users

The existing spell checking system has all the features that most modern browsers have. It can suggest words, can add custom words, supports 27 languages (and counting), can install spellcheck dictionaries without user intervention, can change spell check languages on the 'fly', has 'new' words incorporated in many languages (blog), and is generally known to be stable and useful, without being intrusive. As a slightly more advanced feature, automatic spelling correction was added, as an experiment (under a command line flag). It has been moderately successful, albeit with some small nuances which need to be fixed before it can be fully incorporated as a feature.

At this stage, improvements to spell checking have to go beyond what every other browser provides. One important aspect of spell checking is the fact that many users are multi lingual. They would like to see spell check support which would enable them to switch between their chosen languages of communication through the browser smoothly, expecting the spell checking system to adapt to their usage automatically, in real time. This would require the SpellChecker to automatically detect the intended language of use as the user is typing, and switch to it smoothly and non-intrusively.


This feature has to be designed such that the user involvement is minimum. The user should be able to simply start typing in a text box (email, comment, doc etc) and the spell checking system keeps pace to update itself to the language the user is typing in. The user should be oblivious to the change of spellcheck language in terms of performance while typing, and should be able to mix multiple languages in a single text box to get the corresponding spell checking done automatically. The suggestions to misspelled words should be presented intuitively without cluttering up the context menu.

When the user mixes multiple languages together,the SpellChecker detects that the user is using multiple languages, and starts supporting the languages simultaneously. This means that, when the user types in a word, it is marked misspelled if it is misspelled in all the languages. Only two concurrent languages will be supported. If a new, third language is detected for use, remove the one which was least recently used.

On right click on the misspelled word, the context menu shows sub context-menus on the top, corresponding to the two languages, and the sub context-menus themselves show the suggestions for the corresponding language.

Design principle

Thanks to prior work done to support this features, Chromium already includes a rich set of features to enable this advanced feature. The following features will be used:

Language detection

The Compact Language Detection (CLD) library was initially written for Google Toolbar. It was ripped off from its dependency with toolbar, and was added to Chromium recently. CLD is fully integrated with Chromium, and does not require any communications with any server. It requires a very low sampling size - most of the times it can detect the language from a single sentence. It is not very resource intensive, and returns the language fast if the input is kept sparse. It returns unknown for languages which it cannot detect.

SpellChecker support

The SpellChecker supports the ability to be changed on the fly. Currently, the SpellChecker can handle one language at a time. It has to be upgraded to handle multiple languages simultaneously, and should have the ability to quickly change these languages on the fly. When this feature is enabled, given a word to check for spelling and suggestions, it will do so for the languages it is supporting, and will generate suggestions for these languages simultaneously.

UI Support

The UI has to be updated in several places to support this feature.

Context menu suggestions

The context menu over a text area currently shows suggestions in a single language. It has to be updated so that it can now support sub context-menus to show suggestions for different languages simultaneously.

Sub context menu in Context menu for spell check options

The context menu for a text area has a dedicated Spell Check options sub menu to select language of the SpellChecker. Only one language can be selected through context menu radio buttons at this time. When the auto detect language feature is turned on, this has to now enable check boxes, which means that the languages which have been detected so far will be shown, and two of them will be enabled (by check boxes).

The spell checking system has to remember the languages typed by the user. Thus, suppose the user types in English, French and German in a session. In a new session, when the user starts using a text box, the SpellChecker will be set to a default one, and these three languages will show up in the sub context menu, with only the default one selected.

Languages Options Menu

There should be quick settings to turn this feature ON/OFF from the Language Options menu.

Note: The Options menu for Languages is undergoing a change where the user can simply set the languages used, and not bother explicitly about accept languages, spell check languages and Chromium UI languages. The change in spellcheck language will be reflected in the languages that the user uses. If the language is not set by the user, it is added as a new language detected to be used by the User.

Limitations and possible drawbacks

While this feature itself is exciting, past experience has revealed that such features can be intrusive and counter intuitive for certain edge cases. Some edge cases are the following:

User mixes multiple languages in a single sentence:

Sometimes, users, when constructing informal emails, mix words from the different languages they know. Of course, the fact that multiple languages will be supported simultaneously is the end solution - but how does one detect these languages?

How to decide the default language to start with

Selecting the default language to start with during spell checking can be tricky. Right now, the following heuristics will be used to select the default language of the SpellChecker - the language of the web page if the text box is empty - otherwise, the language of the words in the text box if it is pre-populated.


The spell check tests have to be upgraded so that they are aware of the SpellChecker's ability to spell in multiple languages simultaneously. Automated tests have to be designed to check the ability of the SpellChecker to change language on the fly adaptively with the language of the text being spell checked..