Scenario: You can speak 2 languages namely English and Khmer. English is your Apple Mac OS primary language; thus, all Apps are in English. But you want to use Khmer as default language for some Apps, while want to keep others in English.
Note: This instruction only works with Mac Applications that have Khmer language package installed. At the moment, we only released Khmer localization package for Stickies 6.0.
Well, here is the trick, take Stickies for example:

Step 1. Go to Applications folder, right click on Stickies, select Get Info (or Command + I)
Step 2. Under Languages options, un-tick everything keep only your preferred language.

Step 3. Quit the Stickies App, and restart it. ha ha, that’s it.
See also:
+ How to add Khmer Language to a Mac Application
+ How to translate Mac OS X application into Khmer language
Keywords: change Mac App default language, change Mac App default language different from system language.
រើសភាសាខ្មែរជាភាសាទី១សំរាប់កម្មវិធីMac ណាមួយ
ឧទាហរណ៍៖ យកកម្មវិធី Stickies ជាឧទាហរណ៍
១. ទៅកាន់ឃ្នាបឯកសារកម្មវិធី Applications ចុចខាងស្តាំលើ Stickies រួចរើសយក Get Info (Command + I)
២. នៅខាងក្រោមជំរើសភាសា Languages: រើសយកតែភាសាខ្មែរ ដកភាសាផ្សេងទៀតចេញទាំងអស់។
៣. បិទរួចបើកកម្មវិធី Stickies នោះបងប្អូននឹងឃើញ Stickies ជាភាសាខ្មែរ។



Nice, Good job. the next step should BE Mac OS in Khmer. m waiting to help with that.
ha ha…Mac OS in Khmer? that’s too big bro,
it will come true one day.
I hope so.:D and that will be the day after i won Mega million lottery. hehe