Has the Catholic Church ever attempted a local substitute for the word Amen? The suffix 'Amen' has been in use  in the prayers of virtually all Christian denominations across the world, though the pronunciation differs from place to place. Given that the names of God and Jesus have local adaptations, it is intriguing that 'Amen' has a  universally accepted, uniquely single form. I am, therefore, curious to know if any local church of the Catholic denomination has ever attempted a translated version of 'Amen' in its own  language.
The short answer is Yes. 

Some French speaking communities often employ the Ainsi soit-il ending at the end of some of their prayers. Although the use of Amen is by far the norm, many people who speak French still use this alternative ending to their prayers.


  For those who wanted prayers: here's the "Hail Mary.
  
  Je vous salue, Marie, pleine de grâce. 
   Le Seigneur est avec vous. Vous êtes bénie entre toutes les femmes, et Jésus, le fruit de vos entrailles, est béni. 
  Sainte Marie, mère de Dieu, priez pour nous pauvres pêcheurs, maintenant et à l'heure de notre mort. 
  Ainsi soit-il. - Hail Mary Prayer 


The word Amen simply means so be it.

It would not be a surprise to me if some Native American languages have adopted a local variation to the endings of some of their prayers. Being simply a traditional way of ending prayers, Rome would not interfere with a local custom. Here is an example of the Our Father in Quiripi:


  Quiripi translation
  
  Noûshin aûsequamuk terre
   wérrettepantammunatch wòweztâuonatch kowésewunk
   Peamoutch' kúkkussootúmmowunk,
   kòrantàmmowunk neratch
   sket'ôkke nenar âusequamuk terre,
   Mèsonah èa kèsuk kónkesekatush noméetsounk,
   petúkkenêag akquantamínah nomàtchereúnganansh
   nenar tàkquantaminan ewojek nomàtcherehéaqueàguk,
   Asquonsàkkongonan rame-re mítchemôuretounk,
   webe kûppoquohwhèriggamínah wutche madjk'
   wutche kèkatah kètassootómoonk, 
   quah milkèssowunk quah àíttarwejanúnguesówunk
   michème quah michème: Ne râtch. 
  
  English version 
  
  Our Father, who art in heaven, 
   Hallowed be thy Name. 
   Thy kingdom come, 
   Thy will be done, 
   On earth as it is in heaven. 
   Give us this day our daily bread, 
   And forgive us our trespasses, 
   As we forgive those who trespass against us. 
   And lead us not into temptation, 
   But deliver us from evil. 
   For thine is the kingdom, 
   and the power, and the glory, 
   For ever and ever. Amen. 


Algonquian and Iroquoian Uses:


  Also called "Indian Masses", a number of variations on the Roman Rite developed in the Indian missions of Canada and the United States. These originated in the 17th century, and some remained in use until the Second Vatican Council. The priest's parts remained in Latin, while the ordinaries sung by the schola were translated into the vernacular (e.g., Mohawk, Algonquin, Micmac, and Huron). They also generally featured a reduced cycle of native-language propers and hymns. At present they are rarely used. - Latin liturgical rites


Historically, Soþlice was used in Old English which could be translated Truly/Amen/So be it.  


  AD 995, Old English
  
  Fæder ūre, ðū ðē eart on heofonum,Sī ðīn nama gehālgod.Tō becume ðīn rice.Gewurde ðīn willaOn eorþan swā swā on heofonum.Urne gedæghwamlīcan hlāf syle ūs tōdæg.And forgyf ūs ūre gyltas,Swā swā wē forgyfaþ ūrum gyltendum.And ne gelæd ðū ūs on costnunge,ac alȳs ūs of yfele. Sōþlice.


The oldest Our Father in French ends with Issi seit. 

More examples may be found here using an alternative ending to prayers other than Amen, including Token's Quenya language of the Our Father .