Software Multilingualization

I would recommend to start with english, followed by your own language.
Then go for the big ones like:
spanish, french, chinese,

or by languages you've expect the best ROI.

I'd say 99% of the time, go for English first followed by your native language (unless English is your native language of course). Never ever use automatic translation for localization unless you strive to become a laughing stock.

I would recommend to start with some selected languages based on your experience, analytics data as well as accessibility of translators and machine translation services.

Consider to use an i18n library with support for icu messages, especially if you have to translate into slavic languages with their plural rules.

Simpleen's UI will stay with english, because our translation product is for developers who want to automatically translate their i18n project in up to 12 langauges. We want to provide more translation languages in the future.

depends on your product and if you can see some stats on people using it form other countries... if you have stats it's popular somewhere, you might consider the local languages there - (after some filtering and considering economics there..)
Also if you know other languages, be easier to start there possible
Due there are services that you just pay for translations than you could just make in into some kind of financial opportunity accounting..
RTL is probably hard for most to do, cause it's a design thing, also a bit of up-down texts..

I think commonly people go for the European languages like German and French as strong economy / buying power and reasonably easy translations as in not needing to redesign things..