I think the scenario you are describing is unlikely because most components will come at least with English and if that is your main language it will show that language. But for arguments sake, let's look at it.
So first we have to define what is your main source language. Let's say it is Spanish to make the example valid. Let's say you have Spanish, English and French installed in Joomla. Joomla ships with English but you can unpublish it.
If you install a component that is developed correctly but does not have language files for French but do have one for Spanish you would see the Spanish correctly and the French you would see COM_COMPOENENT_NAME_STRING. It would be relatively simple to translate the language file to French simply by creating the appropriate language file.
If you do, consider sending it to the component developer so they can include it in future releases.
The above only applies to language strings stored in the language files. If the component stores any data in the database it needs to be developed specifically for multilingual use (which most are not) or you need to use a translation component such as http://www.neno-translate.com
If you use Neno it will display the source language of any string (file or database) until it has been translated.
Note: I work for www.neno-translate.com and I am declaring my affiliation as it is required by Stackoverflow, but I would also like to clarify that it is the only solution that can translate any 3rd party component so referencing it is valid. Also it is free and open source!
