1 – There are a couple of Java libraries to deal with internationalization, as awkay/easy-i18n. I would recommend to take the same approach used by the SUT – libraries and code patterns.
Regarding “Do I create separate tests for 125 different clients?”: You should always keep in mind the test pyramid. Depending on the kind of test you are doing, replicating each validation for 125 can be both unnecessary costly and slow down strongly the performance of your suite.
2 – If you are using the Page Object pattern, you can reach a good level of abstraction of the different forms used by these sites and make your tests easy to maintain. Good code review practices will surely increase the quality of the architecture of your automation suite.