In the past, democracy was not well settled in Korea. So, many social leaders strived to spread Democratic idealogy and systems (pl). D.J. Kim and M.H. Noh, who were the presidents (pl) of Korea from 1998 to 2007, were representative leaders in that area (sing). They cried for the reformation of Korean Politics, and fighted  fought with the dictatorship for the country and the people. Thanks to the  their devotion of them, Korean Democracy has been developed, and our level of democratic system became world class. That is, they were good leaders and good citizens for Korea. By the way, it seems that the meaning of 'good citizen' is changing nowdays. The long-time economic recession makes (-S) the people to  think about their own interests firstly and not to be generous for to others (no apostrophe) . In this point, the importance of economics has increased. All businessmen and enterpreneurs that who create economical value for of the society (no commas) are good citizens. For the realization of democracy and the happiness of all people, economic development is necessary.

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"Thanks to their devotion, Korean democracy has been developed"
--The present perfect continuous tense is Has/Have + been + present participle (verb-ing)

Therefore, "Thanks to their devotion, Korean democracy has been developing" would be the correct sentence. However, the use is wrong. They contributed to the establishment of demoracy, but their influence today is probably limited. Also, you don't want to change the tense mid-sentence when possible.

--> Thanks to their devotion, Korean democracy developed and our level of democratic system became world class.
(Developed and Became are the same tense: simple past)

--Others' would be the possessive form. "Others' cars are faster than mine." Your sentence isn't possessive, so no apostrophy.

--The relative clause comma I want to review with you in class. We use commas when the information is extra and unnecessary. We don't use commas when the relative clause is essential to the sentence. Therefore, I left the first one in commas (concerning the presidents) and I omited the second clause's commas (about businessmen) because only businessmen who make money contribute to society. My favorite example is:

-My girlfriend, who lives in apartment 2B, was arrested last night.
--> His gf was arrested last night. Also, she lives in apt 2B: extra info

-My girlfriend who lives in aprtment 2B was arrested last night.
--> This one implies he has multiple girlfriends and the one who lives in apt 2B was arrested.