KDiff3Portable_0.9.98_Development_Test_3

Well, you have some incorrect/missing [LanguageStrings] entries, but even with all of them corrected the option in the app's 'Regional Settings' changes to match the Platform's language, but KDiff3's UI doesn't actually switch to the selected language* :(, so unless someone can figure out a way to make it switch I'm going to call it a bug in the base app and move on. (maybe someone could try getting in contact with Joachim Eibl about it?)

Anyway, here's a fixed version of the [LanguageStrings] section.
Note: Bold items work with this version, but not in DevTest 2.
Note2: this makes SpanishInternational use the Spanish translation, because it's better for it to fall back to (non-international) Spanish in that case than English.

[LanguageStrings]
Arabic=ar (Arabic)
Bulgarian=bg (Bulgarian)
Catalan=ca (Catalan)
SimpChinese=zh_CN (Chinese Simplified)
TradChinese=zh_TW (Chinese Traditional)
Czech=cs (Czech)
Danish=da (Danish)
Dutch=nl (Dutch)
English=en_orig
EnglishGB=en_GB (British English)
Estonian=et (Estonian)
French=fr (French)
Galician=gl (Galician)
German=de (German)
Greek=el (Greek)
Hungarian=hu (Hungarian)
Irish=ga (Irish Gaelic)
Italian=it (Italian)
Japanese=ja (Japanese)
Lithuanian=lt (Lithuanian)
Norwegian=nb (Norwegian Bookmal)
Polish=pl (Polish)
Portuguese=pt (Portuguese)
PortugueseBR=pt_BR (Brazilian Portuguese)
Romanian=ro (Romanian)
Russian=ru (Russian)
Serbian=sr (Serbian)
SerbianLatin=sr@Latn (Serbian)
Spanish=es (Spanish)
SpanishInternational=es (Spanish)
Swedish=sv (Swedish)
Tamil=ta (Tamil)
Turkish=tr (Turkish)
Ukrainian=uk (Ukrainian)

Also these languages are recognised by KDiff3, but not supported by the Platform. I'd suggest adding them to the [LanguageStrings] section to 'future-proof' the app, but I don't know what LocaleName they would use. (this could be done if you switched to use one of the other language variables for [Language]:Base instead, as they follow standards which already encompass more languages than the Platform currently supports.)
"bs (Bosnian)" is 'Bosanski', one of the official languages of Bosnia and Herzegovina
"br (Breton)" is 'Brezhoneg', a Celtic language spoken in the 'Brittany' region of France
"ca@valencia (Catalan (Valencian))" is 'Valencian Catalan', spoken in parts of Spain
"hne (Chhattisgarhi)" is the official language Chhattisgarh, a state of India
"zh_CN_old" appears to be a deprecated version of the Simplified Chinese translation (SimpChinese)
"hi (Hindi)" is one of the official languages of India
"ka (Georgian)" is Kartuli, the official language of Georgia
"rw (Kinyarwanda)" is the official language of Rwanda
"nds (Low Saxon)" is either 'Low German', or one of the related 'Low Saxon' variants
"ml (Malayalammn Mongolian)" is ...? well, I'm not sure if it's 'Malayalam' (one of the recognised regional languages of India) or 'Mongolian' (the official language of Mongolia)
"tg (Tajik)" is the official language of Tajikistan
"cy (Welsh)" is Cymraeg, one of the official languages of Wales

Also I haven't seen any evidence in my (fairly limited) testing that FileWrite 1, 2, 3 or 4 are ever used. Do they get used sometimes? or...?

Footnote: *Actually I noticed that some of the languages (e.g. Arabic) change the context (right click) menu, but I haven't noticed anything else changing.

Edit: You should keep an eye out for <br />, it keeps sneaking into your links & breaking them. As far as the app goes, good job, it looks like you've taken care of everything you can for the moment. I'd like to see the UI switch languages, but AFAICT that's a base app issue, sorry.
Thanks for taking this project on.