How to express "it's about time" in a usual conversation (translating "langsam" (GER) or そろそろ (JPN))

The use of slowly in these sentences is as an adverb qualifying the following verb. It does not qualify the manner or timing of beginning the action of the verb. The construction does not give the meaning that you want.

One way of suggesting a leisurely or slow approach to the action is to use the idiomatic construction “I suppose we should ...” or “Well, I suppose we should ...”

“Well, I suppose we should start eating.” The use of Well, ... refers to the preceding circumstances (that we are here to eat) or conversation.

English often uses deverbal constructions in everyday conversation (for instance, 'she took a walk in the park' / 'he had a shower before breakfast', where 'take' and 'have' are merely carrier verbs, known in such constructions as delexical verbs, bleached of meaning. Contrast the lexical usages, 'she took her dog to the park' and 'he had [possessed] an old Fiat'.)

Using 'make a start [on]' for 'start', and 'make a move' for 'go', are very common hedging devices, conversational softeners (/'oilers').

• [Well], I'd better be making a move [,I suppose].

'Well' (not needing any prior context beyond time spent with the other person/s) and 'I suppose' (hinting at one's reluctance to make the necessary start / departure / ...) are, as Anton says, other common hedging devices (subclass conversational softeners). 'I'd better ...' rather than 'I have to ...' is again softer. 'I must go' is starker, more 'that's the end of the matter' (though sometimes necessary, and nowhere near as dismissive ['hurried!', abrupt] as 'I'm outta here').

• I suppose we ought to make a start.
• I suppose I'd better have another go at trying to understand the difference between raising and control verbs.

The self-deprecating introduction of abstruse expressions / concepts is also usually a convenient strategem for ending excessive dialogue without giving rise to ill-feeling.